TROU DE FER - THE IRON HOLE

Only the best of the best are able to abseil into its depths of Trou De Fer, but even that doesn't guarantee that they will ever manage to get back. That's up to nature to decide.
This is because it's located in the world's rainiest place, on Réunion Island, in the Indian Ocean. Yearly precipitation here reaches a record-breaking 10 meters.
Once, after some huge eruption, the surface of the volcano Piton des Neiges collapsed and slumped one kilometer into its empty interior, leaving behind just vertical walls covered with the most vigorous vegetation one can imagine. The monumentality of the place is emphasized by five huge waterfalls which vanish somewhere deep in the cracks at the bottom of a gigantic drop.
This unusual natural marvel can only be seen from a helicopter, provided that one is lucky and the tropical clouds clear away.

But there's also another option. One that is far more difficult. And that is why five friends have not been able to sleep properly for the last three years.