A fogou (or fougou) is an underground passage or tunnel up to 30 metres (98 ft) in length and 2 metres (6.6 ft) wide. They were constructed in the Iron Age by digging trenches and lining the sides with drystone walling. Roofs were made of flat stone slabs covered with earth excavated from the trench. Many fogous have side passages or chambers They were used from the Iron Age until the Roman period. All twelve known fogous are in West Penwith and around the upper Helford River in Cornwall. They are associated with settlements including courtyard houses, rounds and hillforts. The reasons for building fogous are not known, they may have been refuges, entrances, storage areas or shrines.
The word fogou is from the Cornish language fogow meaning a cave, and was used by the antiquarian William Borlase inhis description of one at Bolleit (Boleigh) near Lamorna.