Updated 19th February 1999; 25th October 2002; 15th October 2003; 1st February 2006; 12th May, 2011; 15th September 2013

The original description from the 1975 explorations reads: An entrance pitch of 110m leads to a second of 35m. Choked at the bottom.
The length of 377m has been been calculated as 117.5m pitch + 90m chamber + p72.5 + p72 + p10 +p15 - the latter 2 from exploration in 2013.

This site is described on the web site of the Sociedad Espeleológica Alto Duero who have explored it as sima M-11. The entrance lies on the western edge of the Hoya de Yusa.
The shaft top is 15 x 12m and drops into an impressive pitch of 117.5m. The bottom enters a large chamber some 90m long by 30 wide and high. There is no plan on the SEAD web site to show the direction of this passage which ascends to where the roof meets the floor. Near the top is a 72m pitch.
At the bottom of the chamber is a 65m pitch that chokes. Other parallel pitches all appear to lead nowhere.

In the summer of 2003, three trips partially re-explored and surveyed the site to produce a plan that shows the cave skirting the north west side of the Hoya de Yusa. There are a number of undescended pitches. The pitch nearest the entrance shaft was dropped to -35m (spits already in). There is a sharp rub point and lots of choss so the final 5m was not dropped. It appears to be a small descending tube at the foot, choked after 3m, heading east from the foot of the pitch.
The "big" shaft on the right near the top of the mud slope was dropped to -35m with a very small tube at the foot and windows into a parallel shaft that appears to be deeper. This could be the SEAD 72m pitch.

At Easter 2011, one trip into the hole started a resurvey and, in doing so, climbed the steep mud slope at the northeastern end to reveal possible ongoing passage beyond a drop. This was explored in the summer, 2013. On this trip, the entrance shaft was described as "awesome" with an alpine chough nest about 50m down and a live adder at the base. The mud slope can be climbed by keeping to the left hand side. At the top are two pitches. On the left wall, the northwest side of the slope, Space Serpent Pitch drops 10m and on the right hand wall, a 15m pitch drops 15m. Space Serpent Pitch lands just above a slippery climb into a small, well decorated chamber. The other drop, SOS Pitch, lands on the floor of a large aven. There is no sign of continuation from either pitch. These finds have been sketched onto the plan of the cave.

References: anon., 1975b (Easter and summer logbooks); Kendal Caving Club and Manchester University Speleological Society, 1975; Corrin J S and Smith P, 1981; Manchester University Speleological Society, 1982; Garcia J L, 1987; García José León, 1997; SEAD website; anon., 2002b (summer logbook); anon., 2003c (summer logbook); Corrin Juan, 2003b; Corrin Juan, 2005; León García José, 2010 (Volume 1 and Volume 2) (survey and photo); anon., 2011b (Easter logbook); anon., 2013d (summer logbook)
Entrance picture: from the 2003 explorations 1  2
on the SEAD website
Underground picture(s): 2003 explorations : 2013 explorations
Video: entrance pitch : summer 2013 explorations (YouTube)
Detailed survey:  elevation on the SEAD website :  plan 2003 : elevation from León García José, 2010 (Volume 1 and Volume 2). (Cantabria Subterránea. Catálogo Grandes Cavidades.)
plan 2013 with sketched additions
Line survey:
On area survey:
Survex file: download (2003 resurvey) (Amended magnetic declination December 2013 to align with Eur79 grid and coordinates altered to fit ETRS89 datum, April 2014.)