As the hit travelogue about the worlds beneath us becomes a film, its maker takes us on a voyage through Las Vegas storm drains and the caves of Yucatán – via Goatchurch Cavern in the bowels of SomersetJust off the B3134 in Somerset is a portal to the underworld. The smaller of two openings to Goatchurch Cavern, it’s called the Tradesman’s Entrance – and through it I am squeezing. After tumbling on my bum over damp smooth rock, lacerating a jumpsuit in the process, I venture down and down, sometimes crawling, sometimes standing upright, trying to find footholds in the dark.I’m here with film-maker Robert Petit, so he can show me something of what he’s been experiencing for the past five years, on his way to making an endearingly poetic documentary film called Underland, which riffs on nature-writer Robert Macfarlane’s bestselling 2019 subterranean travelogue of the same name. We’re heading 100ft underground to the Boulder Chamber where, over sugary snacks, I will quiz him about his obs...