“You’ve got to be a sandwich short of a picnic to give it a go.”

That is the verdict of Tunnel Ultra runner Karl Baxter, who entered a 200-mile marathon in a disused railway tunnel in Bath.

Participants have to run the distance in 55 hours in pitch-black darkness. Many do not finish and one entrant went temporarily blind in previous years.

The challenge is described as a “mind-bending test of extreme endurance and sensory deprivation”.

Video journalist: Alex Howick