The Red de Toneyu is the largest cave in Asturias, with 19 kilometres of galleries and a depth of 614 metres. The water infiltrates rapidly and is channelled through a complicated system of underground conduits.
Successions of shafts and narrow meanders lead from the various entrances to the approximate level of minus 200 metres deep, where the fossil galleries are accessed. The resurgence is the Fuente de la Redonda, located at elevation 590 on the eastern slope of the Sella Gorge, in the bed of a stream that flows into the river downstream from the Vidosa Bridge.
The Toneyu Network was declared a Natural Monument in 2003, and is included in the Picos de Europa National Park.
The Toneyu Network is located in the municipality of Amieva, in the Toneyu sheepfold, in the Sierra de Beza.