Olhos de Água do Alviela

The main karst spring of the river Alviela at the foot of the Monsanto syncline in central Portugal, once tapped for Lisbon's water supply. A French-Portuguese expedition surveyed 1,370 m and reached 125 m depth.

The Olhos de Água do Alviela rise at the foot of the Monsanto syncline and drain most of the Planalto de Santo António karst plateau, with part of the water thought to come from the Mira-Minde polje to the north-east. In the early twentieth century the spring was captured and linked by aqueduct to Lisbon, though it was later replaced by other sources.

The flooded cave descends as a sizeable gallery, 4 to 8 m across in places, with side branches. Patrick Jolivet reached about 580 m and 98 m depth in 1988. A national cave-diving commission (CNPS) expedition of the FFESSM, organised with Portuguese cavers who handled accommodation, re-lining and authorisations, re-equipped the line, surveyed 1,370 m including about 200 m of new passage, and stopped at 125 m depth more than 700 m from the entrance.

The account notes a water temperature of about 17 °C and a depigmented crayfish seen 140 m into a side passage.

The spring drains the Planalto de Santo António; tracing work cited by the expedition indicates that part of the water comes from the Mira-Minde polje.

A major Portuguese karst spring, historically part of Lisbon's water supply, and the deepest documented cave dive in the sources found for Portugal.

Visibility on some dives in side passages dropped to 0.5-1.5 m on the return.

نطاقات اعتيادية من مصادر منشورة، لا ظروف آنية.

One entrance, the spring pool; the expedition rigged a tyrolean to move equipment to the pool edge and a rope deviation for the last pitch.