I’m in a dark corridor with four strangers. The ceiling is low, as is the temperature, and the granite walls are covered in engravings: overlapping patterns of wavy lines and psychedelic swirls, like giant notepad doodles etched in stone. We stumble slowly along the passage, peering at the carvings by torchlight. It feels a bit like being in an art gallery during a power cut – except here, the gallery in question is a 6,000-year-old burial chamber on an uninhabited island.
