Updated 31st October; 4th November 2010; 19th, 27th May 2011; 29th April 2012; 7th October 2013; 24th May 2017; 7th January, 1st July 2018; 8th January 2020

The entrance to this cave lies on the true right bank of the río Aguanaz, about 10m from the large resurgence cave. Even with many years of visiting this resurgence area, the obvious entrance to the cave had been ignored or assumed to be an overflow to Aguanaz cave. Invisible Cave could have been an inlet into the Fuente Aguanaz system before it cut back, but it is now an apparently separate system that heads upstream to the south east through deep water to a sump. A top entrance, Entrada Tres (site 3291), intercepts the stream passage before the sump. (An earlier grid reference - altered summer 2013 after the survey of Aguanaz main and second entrances - was 30T 0446611 4801750.)

Entrance 1 drops into chest-deep water with a walk to Room 306, where a 1.5m step up provides a relatively dry area. Water bubbles up under this step. The crawling route forward leads to a pool that almost meets the roof and effectively seals off any further progress. On the first exploration, this pool was drained and 10" depth of water removed with a submersible pump and a generator at the cave entrance. This allowed flat-out crawling between and beneath boulders into the continuing upstream passage.

To allow a more permanent entry and exit, a boulder choke in the cave was located and excavated on the surface. This fairly precarious, bouldery climb down is the second entrance and enters the cave upstream of the pool and boulders. The entrance collapsed when a team was exiting the hole after the summer 2010 dive. It may need re-excavating. Beyond here, the cave is mainly hands-and-knees crawling past and over gravel banks that enlarges after passing under bone breccia in the roof and run-ins on the left to a substantial stream passage. Just beyond, about 100m from the second entrance, a deep pool is met and a draughting duck under the left hand wall (now lined) is required. (This duck was lowered by digging out consolidated sediments downstream in the larger section.)

The passage beyond has waist- to neck-deep water in gloomy phreatic passage until cobbles and boulders in a rift provide a respite. Climbing up the boulders in the rift ends at "station 14" where, on the initial exploration, a climb up a narrow rift continued above. This is the entry point for the third entrance (Entrada Tres, site 3291) detailed below. Continuing in the deep water to the right of the rift for 60m meets a "dead end" in neck-deep water. A possible continuation upstream was detected under the eastern wall. This sump was dived in 2010.

The first upstream sump 1 drops down about 3m then, immediately, rises into an out-of-depth canal. The obstacle "might be free-dived with care". After 6m the passage splits with an inlet from the north ending at a small sump after 10m. About 20m beyond the junction sump 2 is encountered which starts with a blind airbell and continues with an open sump. The log book account of the second dive through the sump can be found here.
The second sump was dived by Chris Camm for 10m in April 2010 and left open and going.
Sumps 2 through to 8 were dived at Easter 2012 by Dan Hibberts and Martyn Grayson. The dive report covers diving these sumps.
Sump 2 is 10m long and 2.1m deep. At the other side, the passage continues for 150m. Three large avens are passed, one of which has a large inlet 15m above the floor of the passage. Sump 3 is 15m in length and 2.1m deep and a fairly simple dive. Beyond sump 3 a large aven was found almost immediately and around 25m of passage to sump 4.
Sump 4 looks possible to bypass by climbing a slope on the right side of the sump where a large ascending unexplored passage can be seen. Sump 4 is 5m long and 2m deep before surfacing in a sump pool in a rift which is 5m long to where the next sump is met.
Sump 5 An eel has been met in this sump. It is 10m long and 2m deep surfacing into 50m of large stream passage ending at a sump.
Sump 6 descends steeply down a mud slope. A depth of 5.1m reached at the base of a vertical pot. Rising here surfaces in a cross rift which is no more than 5m long before the next sump.
Sump 7 is 6m long and 1.9m deep surfacing at the foot of large aven and another sump.
Sump 8 had green leaves and twigs floating on the surface just below an aven when first inspected. This sump has been dived 15m to a depth 6.6m where the line ran out.
Sump 9 was pushed at Easter 2017 by Jim Lister and Dan Hibberts but found blocked by gravel on a steeply descending slope. The site was excavated by "rabbit kicking" and it may be possible to pass. However, both divers had stomach problems in the night confirming bad pollution in the water. (The account of day 2 (19/4/17) trying to pass sump 9 is in the autumn 2019 logbook.)

The third entrance at site 3291 is a 15m pitch that narrows near the bottom before breaking out into the rift. This has proved awkward to pull gear up. The pitch lands on a steep slope of rubble. Up slope ends at a tiny passage in the right wall and straight down the slope chokes. However, just off to the right, in tall passage, a step over a lip meets a 4m pitch down to a rubble slope and 2m climb down the side of a boulder to a small chamber that chokes straight ahead. To the left a short, but steep, bouldery slope intersects a cross rift that needs investigating in both ends at high level. A ladder climb down the rift (p5) encounters "station 14" in Invisible Cave and a bouldery climb down to the stream.
Down-slope from the entrance pitch, just before the second drop on the right, is a step up on the right. This enters passage that turns back and passes a display of tree roots and a short, slippery back-and-foot traverse over a pit. The narrow passage deepens forcing a 6.5m pitch, belayed from stal in the roof bedding. At the base, a shuffle through pools leads to a squeeze past a small column into a decorated chamber with holes in the floor. Down to the left is a chamber with an aven and 2m deep bouldery drop.

A diagram of the hydrology of the San Antonio - Hornedo - Cobadal area drawn after Easter 2011 can be found here.

Link to entry in the Cave Diving Sump Index.

References: anon., 2009c (summer logbook); anon., 2009d (autumn logbook); Corrin Juan, 2010 (photo); anon., 2010c (summer logbook); Corrin Juan, 2011; anon., 2011b (Easter logbook); anon., 2012b (Easter logbook); Corrin Juan, 2013a; anon., 2017b (Easter logbook); anon., 2019e (autumn logbook)
Entrance pictures: 2009, including working on the 2nd entrance
Underground pictures : 2009 (also see Entrada Tres photos) : 2010 : 2011 Easter : 2012 Easter
Video : yes (10Mb, 3min) : in sump 4 and 5, Easter 2012 : capping at the second entrance, 2009 (YouTube)
Detailed Survey : 2009 : 2010 : 2011 (Easter) : sketch of sump 1 : Easter 2012
Line Survey :
On area survey :
Survex file : yes (Easter 2012) (Amended magnetic declination December 2013 to align with Eur79 grid and coordinates altered to fit ETRS89 datum, April 2014.)
Passage direction rose diagram: 1/7/2018