Fogou as they're called are underground structures, tunnels with dry-stone walls and roofs constructed during the Iron Age in Cornwall. Only a few are known to survive today and their intended purpose is known; it's speculated that they may have been used for storage, drying of meat, refuges in case of attack, and that archaeological all purpose answer 'ritual purposes'. However all these possibilities have problems; they weren't at all hidden, making them unlikely refuges, the dampness would mitigate against food storage, most Druidic tradition focussed more on the living world and they don't seem suited to drying of meat for preservation.
A minor mystery for your players to solve.
Time, time,time, see what’s become of me. While I looked around for my possibilities... My AITAS files.
There was actually a Big Finish audio adventure featuring a particular fogou - it is called "The Spectre of Lanyon Moor". Very good audio featuring Colin Baker, Nicholas Courtney (in his Big Finish debut as the Brigadier!) and of course, Maggie Stables playing Evelyn Smythe, who, in my opinion, beats out a certain wife as my favorite doctorate-bearing companion.