Tresviso Caves Project

Tresviso Caves Project Gazetter

T19: Cueva del Marniosa

Area:Sobra Valley
East, North:361359, 4790462 (nearby)
Coordinate Quality:GPS
Long, Lat:-4.707984002136, 43.254189634855 (map)
Elevation (m):974
Length (m):8443
Depth (m):-445
Active Lead:
Survey Available:


The entrance is about 1.5km from the dam up the Sobra Valley, on the left-hand side as one walks up valley. It is close to, and 50m above two cabanas. Between the entrance and the cabanas, stands an electricity pylon and to the left of an obvious entrance, in a small gulley, lies the entrance to Cueva de la Marniosa.


The entrance is well hidden until one ascends the gulley where a strong draught comes from a fissure, 1.5m high by 1m wide.


Entrance Series

Immediately inside the entrance, a small chamber is encountered with old wires and pieces of wood hanging from the ceiling. At one time this part of the cave was used for storing cheese. After a short low section, the passage continues as a high (10m) rift, probably the old abandoned inlet. A 4m pitch is followed quickly by a 12 m pitch. The cave continues steeply down before a 4m climb / pitch is reached, this leads to into an impressive chamber, Morning Chamber, with many fine formations. Several passages lead out of this chamber, all choke or become too small, apart from one strongly draughting passage at the bottom end of the chamber.

The passage becomes much smaller as one climbs over a mud and calcite floor and enlarges as one nears the head of the next pitch. A 22m pitch lands in a moderate size chamber with a passage leading off that goes to two separate pitches. Both pitches may be by-passed by a slope to the right. A low crawl on right leads to Extra-Caverns Series) The passage continues as a large, well decorated tube, with a mud floor, which in places has calcited over. There are numerous small crawls and side passages that have not been pushed to any conclusion. The main passage continues via a muddy chamber, Smoker's Corner, to a rift passage eventually appearing to close in a blank wall, but a climb 3m (ed. rigged pre-2011) up into a roof passage leads to a continuation, via a smaller, but very well decorated section. The larger passage is met again and soon leads to a 20m (very muddy) rope climb, Papoose Pitch, from the head of which the sound of falling water can be heard. A ledge is reached, from which a climb on the right as one looks downslope, leads straight to the stream via a 7m pitch.

At the bottom corner of Morning Chamber, on the opposite side from the main route down to the fourth pitch is a 1.5m climb leading to a 35 degree calcite slope. The slope is 4m to a high level set of passages, Elevenses.

Turning right the passage continues above Morning Chamber with various holes dropping back into the chamber, the end of the passage is blind.

Left reaches a chamber, with two ways on, right and straight on (slightly upslope), Straight on leads to a squeeze through stalagmites, and into a well decorated gour pool passage, followed by impressive columns in the walls and stalactites hanging from the ceiling and stalagmites in the floor. The passage ends with a blind aven to the right hand side of some impressive flowstone.

Right at the chamber leads to walking passage (with a blind passage on the right, and some other blind passages off to the right and left) until a right hand turn at the end of the passage leads to a squeeze through at almost ceiling height where there is a fork in the passage with one passage on the right heading down into a blind hole and the left being a 20m crawl to a choke (70m above and 20m horizontally from the northern passage in Extra Caverns Series).

Smokerís Aven No 1
On the left of the passage, 35m before Smokerís Corner, there is an obvious alcove with a large false floor about 3m of the ground, this is Smokerí Aven No 1.

As you face the aven there are several stalagmites on the right, which can be used to free climb, until level with the false floor. The original bolt climb begins here and starts with an easy traverse on the false floor, until directly below a calcite flow, coming in from the aven. A 17m pitch up ascends next to the calcite flow passing under further stalactites part way up.

At the top of the aven it narrows considerably to an awkward upwards squeeze and left turn. However, the aven ends soon after and there is no way on.

Smokerís Aven No 2
On the left of the passage, 40m after Smokerís Corner, while traversing over the shallow stream trench, there is an obvious chert / nodule climb. This is Smokerís Aven No 2.

A simple 6m bolt climb leads to a ledge above the main passage. A small alcove on the left looks back down to the passage below. There is no way on.

Extra Caverns Series

The Extra Caverns Series was first explored by L.U.S.S. in the 1970ís and referenced a number of possible leads. In 2018 the area was revisited and resurveyed, revealing some errors in the survey and description, with possible further leads for future exploration.

At the bottom of the main 22m pitch, in the entrance series before the streamway is reached, follow the obvious slope down into the continuing main passage. The start of Extra Caverns Series is a crawl through a low and wide arch on the right. Immediately after the arch the passage enlarges, and it is possible to stand and walk. After 16m an aven (approximately 12m unclimbed) enters on the left. Two low crawls on the right, either side of the aven end quickly. The passage continues for 17m to a low inlet crawl on the left (this has not been explored but is extremely tight). After another 12m an obvious chamber is reached, Mud Chamber, with several ways on.

A few metres back from the chamber an inlet on the right quickly lowers to a series of tight bends that connects back with Mud Chamber.

On obvious mud bank on the left, marks the start of a steep ramp up. This has been climbed to around 45m height, generally following the trend of the passage below and continues unexplored. The lower passage is reached by a 3m awkward down climb. 30m of large impressive passage follows to the base of an easy 11m climb up a calcite slope. The passage continues briefly before lowering into a tight rift. It is possible to push this to small chamber, but there is no obvious continuation.

Ahead at Mud Chamber leads to a short climb down, right connecting back to the previous small inlet. 5m ahead on the right is climb up into passage leading north. After another 5m a junction is met. Left leads to 8m of passage lowering to a crawl, this has not been explored. Right leads to 50m of passage, passing mud banks on the left and an unpromising flat out crawl on the right. The passage ends at a small calcite flow with no way on.

Continuing straight at Mud Chamber there is an obvious steep calcite ramp. This has been climbed to around 40-60m in height and continues and is unsurveyed. To the right of the base of the ramp after 3m a low crawl is reached, this has not been explored.

Following the passage left out of Mud Chamber follows a small immature streamway upstream, with a slight draft. The stream can be followed for around 50m to a sharp right-hand bend. A passage on the left leads after 10m to two possible climbs (unexplored) and the main passage ends after another 10m. Back in the streamway a chamber if reached with an obvious wet aven entering in the right, this is the Free Willy Aven.

The passage continues past a 15m climb up on the right (not explored) for another 45m, passing an inlet on the right, Shit Inlet, which leads to a small old boulder choke. There are several muddy climbs up in the area, but the main route continues to a 4m climb down into a small chamber. There are three avens entering the chamber. The first aven (on the left) has not been explored, the other two avens unite at the top into Forgott Passage.

Free Willy Aven

From the base of the aven, a 5m pitch up (rigged as of 2019) leads to a large eyehole, climb through the eyehole into a continuation of the aven. Down is tight, but only leads back to the base of the aven. A further 6m pitch up (rigged as of 2019) leads to a split in the aven. The left-hand route, Free Left Willy, continues up for another 5m to a narrowing of the passage. The roof of the aven here becomes increasingly calcited and only small tight options are possible. This aven has been derigged.

The right-hand route, Free Right Willy, back at the junction, quickly splits again. Right leads back into the main Free Willy Aven (Free Willy 3). Left at the split, leads to another small aven, with a trickle of water, this is 6m high to a tight squeeze into a small chamber, leading to a further aven of 12m. This has not been climbed but the route has been derigged.

Free Willy 3
From the base of the Free Willy aven, a 5m pitch up (rigged as of 2019) leads to a large eyehole, climb through the eyehole into a continuation of the aven. Down is tight, but only leads back to the base of the aven. A further 6m pitch up (rigged as of 2019) leads to a split in the aven. The right-hand route, Free Right Willy splits and right leads back into the main Free Willy Aven.

An initial traverse leads to y-hang and the start of a P10 (rigged as of 2021) to another y-hang. This is the limit of the bolt climb currently. The aven continues up for at least another 10m with no obvious closing down of the passage. A further potential ledge can be seen a further 5m up from the current limit.

Left at the split, leads to another small aven, with a trickle of water, this is 6m high to a tight squeeze into a small chamber, leading to a further aven of 12m. This has not been climbed but the route has been derigged.

Shit Inlet

Following the main Extra Caverns Series passage, after the Free Willy Aven, the passage gradually becomes more rift like for around 20m and passes an obscure inlet on the right, Shit Inlet. The passage can be followed upstream for approx. 8m to the base of an unclimbed aven (approx. 12m). It is possible to further follow the trickle of water upstream through a low crawl for another 8m at which point the crawl turns sharply left and the passage shrinks to only a few centimetres high, making it too tight to continue.

Forgott Passage

Following the main Extra Caverns Series passage, after the Free Willy Aven, the passage gradually becomes more rift like for around 20m and passes an inlet on the right, Shit Inlet, before entering a small boulder choke. At the end of the boulder choke is an awkward 2m climb down into a further small chamber. The exits from this chamber are several climbs in the roof. The far-left climb is currently unexplored.

The two climbs on the right unite at the top and lead to a tight thrutch in a traverse, approx. 5m above the stream. This is Forgott Passage and leads to a window over a lower continuation, which can be accessed by following the right-hand wall, entering a chamber with an unclimbed aven in the roof. The passage continues upstream, with a trickle of water passing under the window, seen earlier on the higher route, to a short climb to the base of another unclimbed aven.

Not a Papoose Bypass
A roped climb of 3m marks the start of the Moonmilk Crawls, 40m of degraded calcite crawling, that opens out into the start of the passage leading to the top of Papoose Pitch.

On the right hand side of the passage is an obvious hole, with a trickle of water. This can be down-climbed carefully for 5m to a small climb down into immature streamway. This can be followed for another awkward 5m, to the top of further awkward down climbs, that continue down at least another 15m, but have not been explored.

Back in the main passage and another 10m along, on the right hand side again, is another obvious hole. This drops almost vertically for another 15m but has not been descended.

These 2 passages have not been fully explored but are heading down to the mainstream, approx. 15m below.

From the bottom of Papoose Pitch, before heading through the hole toward the main streamway, turn left over a large boulder and then up a muddy slope until you to a 5m pitch up (rigged 2021). At the top there is a short traverse to the left. Straight over the traverse enters a boulder strewn area that continues to a rift section. The way on is along the obvious passage which continues 15m to a junction.

Left at the junction goes to a well decorated chamber floored with boulders. An easy squeeze leads to lower crawling passage that goes under the main passage until it finishes.

Right at the junction leads to a small chamber where there is a higher level and a window into the rift section, beyond the initial traverse. The rift is over 15m deep and the sound of water flow can be heard (most likely the main streamway).

Traversing along the rift for 10-15 m leads to a large window on the right.

This leads to a further passage, on the left, that continues to a crawling passage and end. On the right is a complicated layered set of passages that go under each other and have signs of old collapse. This area is quite well decorated with lots of fossil stal and flowstone. Continuing along the traverse in the rift, it continues northeast at least 20m but has not yet been explored. A rope would be required to rig a traverse along the rift. Looking along the rift a passage with a floor can be seen.


Upstream of the ladder pitch, leading into a streamway, lies a considerable length of passage, perhaps 2-3km, all of which was not surveyed. The going is slightly more difficult than downstream, the passage being narrower, more restricted, and containing considerably more roof-fall. The first collapse is 100m from the ladder and blocks the way completely.

Boulder Choke 1 (Immolation Chamber)

Approximately 2 minutes upstream in Cueva de la Marniosa the first boulder choke is reached. Immediately before this there is an awkward climb up amongst calcite. This leads to an aven which makes it way up through the boulder choke and is very loose. Above this the chamber breaks into a large rift (approx. 15m x 20m in places) which is almost certainly older mature passage above the immature stream-way below.

In the downstream direction there is another drippy aven which was not explored. Upstream the rift drops down a muddy ramp to a short pitch which most likely re-joins the boulder choke below. Traversing around this pit however continues along the rift to another pitch (undescended). The continuation of the stream-way can be seen below.

The main route on is up a climb through boulders and over the fall. Many of the boulders seem precariously perched and the smaller ones move under foot. Small inlets enter at the point of many of these falls and often there are large black holes in the roof. Climbs in the roof of these area many be very rewarding.

The upstream continuation can be followed all the way to an immense chamber, called The Hall of the Mountain King. It consists of numerous connected avens, some with waterfalls, others dry and could be anything from 150' high. The floor is littered with sandstone boulders, suggesting that there is a route through to the surface nearby. Probably this chamber is near the sinks in the upper Sobra Valley. (see No. 20 Numerous sinks) There was evidence of flood debris 2.5m up the walls, probably from spring melt.

Yorkshire Inlet (Ayes-For Aven)

Follow the main Marniosa stream way upstream for 700m until two obvious climbs are reached. The second climb of 3m marks the start of the Yorkshire Inlet passage, on the right of the passage, just above the stream way. The easiest way is via an obvious 1m high passage, a low crawl on the right, 3m further along, is a flat out crawl in water that connects to the same passage. The main passage is largely easy walking, in a small stream, for 20m to a sharp left bend. The passage continues for another 40m before another sharp left, followed by a sharp right. After another 20m the passage height starts to dramatically increase. A junction is then met, left leads to a flat out crawl in water (possibly sumped). This was unexplored.

The height of the right hand passage increases further, terminating in the Bradford Aven, a 50m high aven with a small amount of water entering. The Yorkshire Inlet passage heads almost directly north, under the Sobra Valley and the top of the Bradford Aven lies approximately 100m south and 300m below the small obvious peak at the head of the valley. The next obvious right hand passage, between Yorkshire Inlet and The Hall of the Mountain King is the start of the connection with T20A Silvestre Pot.

Harrison Chamber and connection to T20A Silvestre Pot

A climb down over calcite leads to Harrison Chamber, with a 2m climb up the way on. A long straight section leads to the base of a P20, entering from Silvestre (see T20A Silvestre Pot)

Alien Weaponry

From Corner Chamber, in upstream Marniosa, head approximately 125m further upstream to where the passage narrows to a stooping size passage in the stream. Just before the stream bends to the right, is station 42 of the survey, with an obvious dark void above and to the left. This is the entrance to the Alien Weaponry chamber.

A steep climb up large boulders enters the large chamber, with dimensions of over 100m long x 50m wide x 25m high, much larger than the streamway below. The chamber slopes steeply down to the streamway and is unstable in places. Directly ahead and up, from the entrance to the chamber is a small passage that branches into three further passages. All involve hands and knees crawling and are unexplored.

Heading east (left on entering) in the main chamber leads up a further slope to a drop down across further boulders to the sound of the stream. The main-streamway is below and to the left but would appear to be inaccessible from this location. Straight ahead is a low boulder filled crawl with further sound of water, but this is heading directly towards the main upstream stream once more. To the left leads up steeply to the impressive 35m high Ted aven. The aven is 5m in diameter and has a small amount of water entering (2019). It is currently unclimbed.

Heading west (immediately right on entering) in the main chamber, leads to a climb / scramble up to District 9.

There are some possible high-level leads on the north side of the chamber (directly above the stream)

District 9

The climb up from the western end of Alien Weaponry, leads to District 9, this is a continuation of the Alien Weaponry chamber and consists initially of a broad flat floor for 10m, at which point the passage starts to bend left with a steep slope leading to a ramp of 20m leading to a ledge above The 42 Streamway. Staying high and right the main passage continues for another 40m to a further steep ramp down to a streamway below (presumed to be an unexplored part of The 42 Streamway, but not confirmed). There is an unexplored inlet/outlet on the right and on the left is a connection through boulders to Weyland-Yutani.


At the end of District 9, before it steeply slopes back down an opening on the left, through boulders, leads to a further chamber, Weyland-Yutani. A difficult climb down of 13m on small loose gravel and a further down climb (a bolt and handline useful for return visit) reaches the bottom, traversing above a stream. Care must be taken on the traverse as multiple stalagmites and stalactites are around, including a 3í high stalagmite.

The chamber can be followed further, around numerous large boulders with an intense rumbling sound from below but there is no obvious way on from this point.

The Predator
Passing through District 9 and shortly before the boulder ruckle into Weyland-Yutani is a huge boulder. Directly behind this boulder is an open chamber, access is attained via an +8m climb up a jammed boulder. Care should be taken here!

A rope is in situ here, tied around a reasonably good boulder belay (2021). The chamber at the top of the boulder slab is formed as a large aven/ramp with a blind pit on the left hand side. The way on is up the easier angled ramp, there is a trickle of water coming down here and this aven series will probably get quite unpleasant in the wet.

A +20m pitch leads to a small alcove (assumed to get wet) and a further +5m pitch takes you to the less secure upper slopes. Follow the rope with a step out here onto a precarious ledge and a very gentle prussik up leads to an ascending passage.

Float up this, preferably without touching the floor, to a final aven. Don't look up here, instead close your eyes and think brave, slow thoughts as you prussik the final rope into the middle of several thousand tons of hovering rock. This pitch ends in a boulder chamber, hopefully you won't.

There is one lead, back on the ledge, it's possible that by traversing further away from the alcove there is another, slightly smaller aven entering on the far side.

The 42 Streamway

From Corner Chamber head approximately 125m further upstream to where the passage narrows to a stooping size passage in the stream. Just before the stream bends to the right, is station 42 of the survey, with an obvious dark void above and to the left. This is the entrance to the Alien Weaponry chamber. Below the entrance up to Alien Weaponry is a large boulder, climbing over this leads to a short drop down into the start of The 42 Streamway.

The streamway begins as a walking rift passage with a flat sandy floor. After 50m, the right hand wall gives way to a sand and rubble choke with dark space above. This is assumed to be District 9, the survey certainly indicates it is likely, but not confirmed. Climbing up the sand wall proved impossible due to its affinity with the floor. Back in the stream and just around the corner, a muddy ramp is reached. This rises to a T-junction and a draft is present. Right quickly closes with a few loose boulder chokes in the ceiling (possibly the floor of District 9 once more). Left crosses over the streamway below into a fair sized boulder chamber, the passage continues both up and downstream in a rift, both are thought to quickly re-join the streamway below but this has not been confirmed.

Continuing along a streamway, past a small dry oxbow on the right, leads to a squeeze. Beyond this the passage continues as a high, narrow rift canyon with several climbs and static pools. After 100m it begins to close, some progress would be possible, but it would be miserable in wet, small passages and there is no draft. A possible continuation again becomes tight.

Approx. 5-10m back from the pool a 4m climb up leads to a further climb of 2m and into 100m of canal sections, a climb of 4m enters a round chamber. The water (and draft) comes out of a small circular hole 5m off the floor. This leads into the Sardine Streamway.

At the top of the first C4 listed above a high rift passage can be followed 'downstream'. This was followed over several pits in the floor to a logical end point where traversing required bolts. It is assumed to connect to the boulder chamber mentioned above

Sardine Streamway & Tinned Fish
The 5m pitch up at the end of the 42 Streamway, marks the start of the Sardine Streamway.

A small active streamway runs a gently meandering course for a hundred meters or so where a small side chamber enters with a short climb at the back that quickly becomes too tight. Further along the stream passage the height varies from well above head height down to crawling height where in high water the passage looks likely to sump. The passage eventually breaks out into a large chamber with a sloping ramp on the right, leading up to a couple of tight climbs at the back.

The continuation along the streamway, Tinned Fish, eventually reaches a further chamber with a ramp on the left leading up to a calcite choke.

The passage at the end of the chamber splits in two, the right fork, which has most of the water flow, comes through a wet crawl which can be drained to make it more pleasant. This eventually leads to a complex boulder choke with no apparent way on despite extensive exploration.

The left branch goes past some awkwardly placed calcite formation before becoming standing passage. After a 3m climb up a small oxbow is reached with an 8-10 m clean washed aven that would require a bolt climb on dubious looking rock to gain access.

The lower passage eventually becomes too tight to continue.


Downstream a fine high but narrow streamway leads to a wet 4m pitch. After this the going is easy with potholed floor and chert bands in the walls. Eventually a pitch is met, where water descends through an eyehole. This may be by-passed by easy traversing at a slightly higher level. After about 50m there is a 4m ladder climb back to the stream. The passage now becomes more difficult to negotiate, with many chert bands and nodules as the stream descends a series of small falls. the passage eventually gets larger, just before another pitch. It is somewhere before this pitch that the strong draught, which is evident throughout the rest of the system, disappears. It may well be worth exploring the roof (30m above) in this section for this reason.

The pitch is 10m long and afterwards there is a large pool. to the right, a chest deep section leads to a muddy ramp and at its top, phreatic passages lead to a small chamber. A very loose climb, the possible continuation, was not passed. To the left a series of chambers lead, with wading and swimming, to the final sump chamber.

Sump 1

(1986) A 35m sump leads to 650m of spectacular stream passage ending in a 15m pitch. At the bottom of the pitch a deep pool is crossed, but tight rifts lead to another sump. Opposite the 15m pitch. a climb leads to a further 600m of stream rift was found, terminating in Sump 2.

(ed 2013: Sump 2 in the Marniosa - "1986 Extensions" is approx. 500m horizontally and 50-100m vertically from Agua, halfway between Parting Friends and the Far Upstream Sump)

Sump 2 & Beyond

At the end of the 1986 extensions, beyond Sump 1, is Sump 2. The large clear sump pool gradually dips down to a shallow 5m depth, where it continues at an elbow in the passage. The sump continues for approx. 25m (2m high by 4m wide) to surface at a a gravel slope and streamway, approximately 3m wide and 5m high initially. This continues with similar dimensions until a chamber is reached at a 'junction' with several possible ways on. The left passage, follows the water. Unfortunately, the passage reduces in size, eventually narrowing to a choke after around 15m. This choke can be entered for a few metres but its stability is questionable.

Back at the junction, a climb up on the right (slippery) seems to approach a dead end, but a climb back down on the left (care required) bypasses the choke reached in the left-hand passage and re-enters the streamway, which reaches a second choke after ~20m. During this climb down, a side passage is passed at head height on the left (crossing over the choked passage below) and is the main continuation.

Following this side passage leads to a large passage, which can be followed for around 50m, via a few small down-climbs, until a 6m pitch is reached. This pitch is undescended, but seems to re-join the main streamway beyond the final choke. The passage appears large, heading SW.

Note: Rigging this pitch for future exploration will require bolts as the 'naturals' are a little suspect. In addition, despite being fairly high up, this pitch clearly takes significant flow evidenced by stagnant pools along the length of the approach passage.


"Expeditions to Tresviso and the Picos de Europa in Northern Spain (1974-1977)". LUSS (1978).
"An Expedition to the Picos de Europa Northern Spain".LUSS. (1979).
"Agua '86 and '87". SWCC. (1987)
"Tresviso 2011". SWCC (2011)
"Tresviso 2016", Tresviso Caves Project (2016)
"Tresviso 2017", Tresviso Caves Project (2017)
"Tresviso 2018", Tresviso Caves Project (2018)
"Tresviso 2019", Tresviso Caves Project (2019)
"Tresviso 2021", Tresviso Caves Project (2021)


Marniosa and Silveste (Plan)