Tresviso Caves Project

Tresviso Caves Project Gazetter

T145: Pozo del Castillo

(Castillo I)

Area:Mazarrasa Mines
East, North:360524, 4785381 (nearby)
Coordinate Quality:GPS
Long, Lat:-4.7169815908335, 43.208302523077 (map)
Elevation (m):1870
Length (m):1985
Depth (m):-310
Active Lead:
Survey Available:


From Caseton de Andara go towards the Pozo de Andara (Lake Depression), then take a left on the path which goes through the Mazarrasa Mine zone.


The entrance is the highest (T145), it is very easily located by its large dimensions (8m by 15m).

Note: Segura 2 entrance is easiest and safest way in


The large (8m x 15m) open entrance is 13m deep to a floor of boulders and a snow plug in the middle. An obvious opening on the southern wall drops down to the second pitch of 15m with more modest dimensions. Over the top of the second pitch are numerous mine workings that require re-exploration and surveying.

At the bottom of the pitch, straight ahead leads up a short climb to two flooded passages, turn right down a mine passage, with wooden steps, for 20m to a junction and turn right again. The routes straight on and up are unsurveyed and should be revisited. There is approx. 1km of passage in this area.

After another 30m the main Pozo del Castillo junction is met. Straight on leads to the Segura 2 entrance and right leads to Pozo Natacha.

Left at the junction is the main way into Pozo del Castillo, the mine passage descends downwards past stacked deads and contains a strong draught. Two passages on the left, after 10m and 40m are unexplored. The passage turns and trends north for 25m before turning left. A 3m climb on the right leads to a small chamber with a deep pool. On the far side of the pool the in mine passage continues, half flooded, this has not been pushed to a conclusion.

The main Castillo passage continues and passes a small crawl on the right, this leads to a descending tight boulder filled passage for 5m to a sharp left turn. The passage then goes up via some small climbs in mine workings. Not explored.

After another 20m, in the main passage, another small crawl on the right is the way on into Pozo del Castillo. Continuing straight on leads over a spoil pile into steeply ascending mine workings. This was not explored, but is assumed to be the Segura 1 entrance and mine workings.

The crawl is flat out and leads to a small chamber with a snow plug and large propped boulders above. A small squeeze on the right of the snow plug leads further into the chamber, but since the early 1980's no one has passed this point, as it is believed to have collapsed considerably since the initial exploration.

In 2018 the blockage was passed, however, the way on remained elusive. It was possible to traverse around the side of a large snow plug (same as FT16 entrance snow plug), revealing a few unsurveyed side passages and a connection to the lower series of FT16

The floor of the chamber can be reached by sliding under the snow plug for about 10m, then a new drop reaches a rift, 2m wide by 3m high, The Walk of the Metamorphosis. This leads to a pitch of 6m where the draught is very strong. The bottom of the pitch quickly leads to another 12m pitch, with snow on one side, to another pitch of 30m in an elongated shaft of 10m x 20m. There is possible continuation over the top of the 30m pitch.

At the bottom there is another snow patch leading steeply down to two successive pitches of 7m and 12m into a circular chamber of 15m diameter.

The cave then takes a sharp 90 degree bend continuing along a lightly decorated rift before enlarging at the top of a 40m pitch. There a series of parallel pitches over the top of the 40m pitch that connect back at the base.

At the bottom of the 40m pitch is a 3m climb, followed by two superb cylindrical pitches of 7m and 10m. The cave drops another several metres before continuing in a large passage, The Whistling Meander, 2m wide by 5m high for 100m. The cave then starts to narrow to a small trickle of water coming in from an impenetrable inlet and downstream leads to a final rift, Thatcher's Squeeze at -293m.

There is a loud sound of water from beyond the final rift and a very strong draught throughout this section.

FT16 Connection - Basket Case
From the main Pozo Castillo main level, a low crawl on the right, leads to a small chamber and an obvious window, with a boulder and snow collapse. A 5m pitch descends under rubble and at the side of a huge snow-plug. The snow plug is the same one as seen from the surface down the FT16 and FT17 shafts and entered part way down, if continuing along the Castillo mine passage to FT16.

Traversing round the snow-plug (tight in places) gently slopes down and around for around 10m, passing two obvious side passages

The first passage is Basket Case. The passage extends for around 50m, with some old miner's baskets present at the end. There are a couple of unexplored leads in the area, although they would all appear to be heading back up and probably connect with the upper Pozo Castillo levels. Approx. 10m along the passage there is an obvious tight rift on the left, heading down. This appears impassable but a good draught does emit from the rift. To the right is an unexplored mine passage. Straight on continues for around 30m, past the baskets to an obvious slope up filled with mine spoil.

The second passage is around 20m long and leads to a junction, one immediately ends, the other is a steep 5m scramble to another dead end.

The route round the snow plug continues down to an obvious chamber with 2 ways on. Left leads back around the other side of the snow-plug to the 5m. Right leads to the sump at the bottom of FT16. It would appear that the route back up to the FT16 entrance is currently blocked with snow.


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Nichols, T. (1988). "Tresviso '87. To Sara or Bust?". Caves & Caving-39:29-31.
Anon.. "Tresviso '87. Expedition Report" LUSS expedition to Andara, Picos de Europa, Northern Spain. 14 pags.
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Caves & Caving No 39
Tresviso Caves Project 2016, 2017, 2018