From W Hal’s ‘Compleat History of Cornwal, general and parochial’ (1702), quoted by Jo May in his ‘Fogou’ (1996).
[you see] the downfalls of a castle or treble entrenchment in the midst of which is a hole leading to a vault underground. How far it extends no man living now can tell, by reasons of the damps or thick vapours that are in it, for as soon as you go an arrow flight in it or less your candles will go out or extinguish themselves for want of air.