Fogou

2009, Patricia Monaghan, fogou, entry in The Encyclopedia of Celtic Mythology and Folklore, page 198, Underground stone chambers found in Cornwall, believed to have been built between between 500 B.C.E. and 500 C.E., fogous appear to have been places of ritual. As there is evidence of occupation by Celts in the latter part of that period, it is possible that the fogous were built by or used by them, although evidence suggests a pre-Celtic spiritual vision.

Cornish fogous typically consist of a long passage with walls built up in horizontal courses of rough granite stones, typically some 40 to 50 feet long, 6 feet in height, and 5 to 6 feet in width, constructed in a deliberate curve.

All known fogous are associated with settlements, and their creation would have taken considerable effort, so they obviously had some function for these communities. The fogou at Carn Euny dates from the early part of the Iron Age; here it is thought that the large, stone-lined chamber, which is circular and 5 metres in diameter, was constructed first.

Because as you are going to discover very shortly, what's under there is no Bronze Age chieftain, no medieval king, or whatever else you expect to find in a fogou like this.