Dougal Jerram and Michael Usher have just returned from a spectacular and perilous journey to the centre of the earth, to a magical place known as the Crystal Cave.
To reach it, they descended underground, deep into the deadly heart of a Mexican lead mine.
The furnace-like heat and soaring humidity there can kill you in minutes.
But when you see what they saw, you’ll understand why it was worth every suffocating gasp for breath.
MICHAEL USHER: Naica is a tough and dusty mining town in the middle of northern Mexico’s Chihuahuan Desert. Few foreigners come here but it’s where we begin an incredible journey to one of the best-kept and most breath-taking secrets on the planet. Buried deep inside this working lead and silver mine is an extraordinary geological wonder. We’re literally driving down into the belly of this mine now. It’s taken us two years of negotiating to get to this point. Access is rare to this highly guarded treasure. They’re not great places to be if you’re claustrophobic, are they?