The Tresviso Caves Project exploration area is the Eastern Massif of the Picos de Europa, Northern Spain.
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 areas, the Western, Central and Eastern (Andara) Massifs.
The Western Massif is to the west of the Rio Cares and has been explored by a selection of 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).
The Central Massif is between the Cares Gorge and the Rio Duje and has been mainly explored by the Speleo Club de Seine (SCS).
The Eastern Massif is to the east of the Rio Duje. LUSS were exploring the Eastern Massif from the early seventies to 1987 with the help of SEII. It is believed most of the caves drain into the resurgence cave called Cueva del Agua (Cueva del Nacimiento), below Tresviso and at an altitude of +480 metres.
The highest cave entrance, with substantial development in the Eastern Massif, is CS-9 Torca Jou Sin Tierre, at an altitude of around 2070m and some 10 km away from the resurgence.
Prior to 2014 the deepest cave in the area was Sima 56 (Torca del Cueto de los Senderos ) with a depth of -1169m. CS-9 Torca Jou Sin Tierre surpassed this recently and reached a depth of -1203m.
Cueva del Nacimiento itself is over 12km long and currently rises +534m in altitude. The bottom of Sima 56 and CS-9 are lower than the top of Nacimiento.
Sima 56 – Cueva del Nacimiento would be a -1495m deep underground traverse, this would make it the 13th deepest cave in the world (as of 2017).
Torca Jou Sin Tierre – Cueva del Nacimiento would be a -1594m deep underground traverse. This would make it the 8th deepest cave in the world, the deepest in Spain and the 2nd deepest traverse in the world (unconfirmed)