At 200 miles, it might not be the world's longest race, but Tunnel Ultra is certainly the darkest. It's a gruelling two-day endurance event where competitors run underground laps in single-file, to a backdrop of classical music. Headphones are banned but hallucinations are all but guaranteed, with sightings of abominable snowmen, giant slugs and orange blobby monsters all reported. So why do people do it? This is the story of those who do, including one who suffered temporary blindness... then thanked the race organiser for the privilege.

The Never-Ending Tunnel has been adapted from an article by Justin Goulding on the Insight pages of the BBC Sport website. Audio scenes have been re-created for your enjoyment.

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