Tresviso 2023

Eastern Massif, Picos de Europa, Northern Spain

Tresviso 2023

In just under 2 weeks the members of the 2023 expedition, to Tresviso, will be leaving the UK by various means to travel across to Spain.  All being well there will be blog updates from both the top camp and bottom ‘camp’.

Overview & brief history:

The Picos de Europa is a range of mountains 20km inland from the northern coast of Spain, forming the westerly extension of the Cantabrian Mountains.

It consists of three main ranges, the Western, Central and Eastern Massifs.

  • The Western Massif (Cornion) is to the west of the Rio Cares and has been explored by various clubs including Oxford University Caving Club (OUCC), Seccion de Espeleologia Ingenieros Industriales (SEII), York University Cave and Pothole Club (YUCPC), Sociedad de Espeleologia Geologicas (SEG) and Speleo Club de Paris (SCP).  This includes the area under exploration as part of the Ario Caves Project and includes the Pozu Xitu – Cueva de Culiembro system (-1264m)
  • The Central Massif (Urielles) is between the Cares Gorge and the Rio Duje and has been mainly explored by the Speleo Club de Seine (SCS), Spekul and IE Valencia.  Caves in this area include Sistema del Trave (-1441m), Pozo de la Cornisa (-1507m) and the deepest cave in Spain, Sistema del Cerro del Cuevon (-1589m)
  • The Eastern Massif (Andara) is to the East of the Rio Duje. Lancaster University Speleological Society (LUSS) were exploring the Eastern Massif from the early seventies to 1987 with the help of SEII, before several Spanish clubs, including AD KAMI, GE Flash and Tracalet, took over exploration from the early 1990’s.  The major caves in this area are CS-9 Torca de Jou Sin Tierre (-1203m) and Torca del Cueto de Los Senderos (-1169m) and the main resurgence, Cueva del Nacimiento (+535m)

It was the Eastern Massif that drew the attention of LUSS in the early 1970s. In the early years exploration was mounted from the mountain village of Tresviso and exploration was dominated by the resurgence cave Cueva del Nacimiento (Cueva del Agua) located at the foot of the mountain range.

The resurgence flows into a canal that contours the Urdon gorge with an average flow of 2 m3/s and the passages behind the resurgence lead to >12 km of surveyed passage and a height gain of over +535m.  The catchment is some 40 km3 and includes caves such as Torca Jou Sin Tierre (CS-9) at -1203m deep and Torca del Cueto de Los Senderos (Sima 56) at -1169m deep.

In the late 70’s LUSS pushed Cueva del Nacimiento to around the 11km and +300m mark, but after several years the cave was abandoned as all the major routes sumped and no continuation could be found. LUSS then turned their attention to the Andara region higher up the mountain range and sought caves that would drop into the Nacimiento system and hopefully create a record-breaking >1500m traverse.

They found several deep caves around this ‘top camp’ area; Sara, Tere, Flowerpot, Dosser’s Delight and Sima 56 were all pushed to respectable depths, but a connection remained elusive.

LUSS and then SWCC spent some time diving the sumps in Nacimiento and later 90’s trips concentrated on the heavily wooded Sierra del a Corta region, looking for a potential middle entrance between the resurgence and the deep caves at the top of the mountain range.

The Spanish clubs, starting in around 1993, based themselves out of the nearby village of Bejes and gradually explored out towards the same area as LUSS, sometimes discovering new ways on in existing caves or finding new deep systems, culminating in finding the deepest in Andara, CS-9 Torca de Jou Sin Tierre (-1203m).

In 2010 UK clubs, featuring ex-LUSS cavers, started returning to area, exploring the furthest reaches in Cueva del Nacimiento once more, and re-visiting some of the deeper caves, as and when caving numbers allowed.  Collaboration with some of the Spanish clubs became the norm and the Tresviso Caves Project started, a joint undertaking across numerous UK and Spanish clubs.

The overall aim remains the same, a connection between one (or many) of the deep potholes to the resurgence, creating a cave system in the top 10 of worlds’ deepest caves, the deepest in Spain and potentially one of the deepest through-trips in the world.

The depth potential to Cueva del Nacimiento, at an altitude of 470m, from a selection of the major caves on the Andara range:

Top EntranceAreaAlt (m)Current Depth (m)Vertical Range (m)
CS-9 Torca Jou Sin TierreCueto de los Senderos2074-12031604
Torca del Cueto Senderos (Sima 56)Cueto de los Senderos1975-11691505
S33 Torca de la HendidaSamelar1975-4521505
Sistema Sara (from T78 entrance)Vegas de Andara1884-6441414
Sistema Castillo (from T145 Pozo Castillo entrance)Minas de Mazarrasa1870-3091400
2.24 TEREVegas de Andara1820-7921350
Sistema Ramazosa (multiple entrances)Minas de Mazarrasa1820-3131350
Torca del Picu Boru (T169 Flowerpot)Pico Boro1785-7231315
Torca del Cueto de Los Calabreros (T173 Dosser’s Delight)Pico Boro1706-8311236
T190 Torca SeptrinPico Boro1696-1801226
Torca de Branaredonda (Fallen Bear)Samelar1589-4561119
T225 BromistaValdediezma1375-130905
T742 Sima Vuc PotSierra del a Corta1320-63850
T510 Cueva del Entre CuetosSierra del a Corta1305-117835

One of the main challenges the expedition faces is that most of the deeper caves have not been visited since the late 70’s to early 80’s.  Although surveys and descriptions exist (of varying quality), there is no way around the fact that those caves will need re-exploring and re-rigging, just to get to the previous limit of exploration before any new exploration can begin.  This requires a tough sell of repeating old work before getting to the exciting stuff!

Tresviso 2023

The 2023 expedition will continue with the exploration of the resurgence caves around Tresviso, specifically the newly discovered ‘way on’ at the end of Cueva del Nacimiento, high level climbing leads in Cueva de la Marniosa and an attempt once more will be made on the streamway leads in Marniosa, beyond sump 2, not visited since 2018.

2023 also sees the return of a large group of Sheffield cavers, after a 4 year break, with the intention of continuing their 2018 work in Torca del Picu Boru (T169 Flowerpot), on the Andara plateau.

A small selection of the objectives:

Cueva del Nacimiento – Exploration at the far end.

  • In 2022 the previous limit to exploration (a small calcite hole) was dug out and passed to over 400m of new passage. This was left with multiple ongoing leads. The passage has hit an obvious fault (shown on all the geological maps) and lies in a prime position for connecting with some of the potholes on the Sierra del a Corta, the middle ground to a further connection with the deeper holes on the top of Andara.
  • Also, at the far end of the cave, but down at the water level, a steep ramp was climbed to ~ 100m to a large chamber and a continuing ramp. These will be revisited.
  • A return to the Far Upstream Sump, last dived in 2012, is planned in 2024, dive bottles and support kit will be moved to the far end of the cave in 2023, in preparation for next year’s attempt.

Cueva de la Marniosa

  • Downstream Marniosa leads to Sump 1. This was passed in the 80’s to Sump 2. Sump 2 was finally passed on the 2017 expedition and was left ongoing at a P6 down into the continuing streamway.
  • In 2019 a dye trace confirmed this water reappears in the Nacimiento upstream sump. The completely different size and scales of passage / water between the two sites suggests something else in between the two (i.e., intersection of major underwater trunk passage). An attempt to push further into the streamway, will be attempted by a team of divers.
  • Before Sump 1, in the main cave, are several high-level climbs above the streamway.  These will be scaled to attempt to find a bypass to the sump.

Cueva C29

  • Permission has been extended into the Invernales de Caballar region (Asturias), a small area to the edge of the current explorable zone. 
  • This includes C29 a cave not visited since 1975, and although only ‘small’ at -158m, it potentially sits between the resurgence and the deeper caves.  Depending on the suitabity of the ‘sump’ at the end, it will hopefully be dived and subsequently dye traced, to prove or disprove a connection with Cueva del Nacimiento.

Torca del Picu Boro (T169 Flower Pot)

  • In 2018 SUSS passed the previous 80’s LUSS limit in one of the branches of the cave (the BILL Series) and this was left at a tight wet continuation. This will be revisited.
  • Survey the BILL series, including the new discoveries from 2018, as this was not completed last time

Dye Tracing and Water Studies

  • Permission has been extended into the Invernales de Caballar region (Asturias), a small area to the edge of the current explorable zone. This area contains Torca La Barga and C29, two caves that contain sizable streams. A dye trace is planned to establish if they are part of the connection to Cueva del Nacimiento.
  •  Also, in 2022 several TinyTag™ temperature sensors were installed in all the known resurgences and have been monitoring for the past year. These will be replaced, and the data correlated with rain fall statistics for the same period.


  • In 2021 the resurveying of Cueva del Nacimiento was restarted. Over the past few years, it has become apparent that a lot of the old data and drawings is widely inaccurate in some places. Modernising the surveying and rechecking old leads has become a prime goal of the expedition.
  • Alongside the exploration of new passage, one of the ongoing aims of the Tresviso Caves Project is to collate and document the area, providing an online database for all the caves in the region. To date >80kms of cave across 1100 sites have been described, logged and made publicly available along with relevant surveys, descriptions and possible leads. ( &

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