Bear Grylls meets two men who spend their lives avoiding confined spaces – and takes them deep into the most frightening caves he can find. Mark almost died when he was buried alive by sand as a teenager. Meanwhile, Mike was accidentally imprisoned in a fairground ride dangling almost 20 metres off the ground. He now fears being trapped so much that he can’t even hug his wife.

Bear has a plan to help them face, and hopefully overcome, their worst fears. He’s heading out to one of the First World War’s most treacherous front lines, where the landscape is still punctured with trenches, caves and tunnels. But Bear will need to win Mike and Mark’s trust as he takes them deep underground.