[Macfarlane and Borodale have been caving in the Mendips]

It is full dark by the time Sean and I get back to the cottage.  We hose down and then hang up our hazmat suits in the cool air of the garden, one on each wing of the totem pole.  I whistle a song from Rubber Soul by the Beatles while we work.

Sean tells me about how he once climbed a wooded slope in Burrington Combe, opposite Aveline’s, and found an entrance to a chamber, the aperture big enough for him to put his head into but too tight to admit his body.

‘I called out into it,’ says Sean, ‘and the chamber answered, singing a different note back to me.’