Sa Fosca — "the dark one" — is the king of Mallorca's canyons and routinely named among the finest in the world. Dropping out of the Gorg Blau reservoir, it is the only canyon on the island with a long underground section: for nearly two hours you move through 300-metre-high walls that close to as little as 40 centimetres apart, blocking out the sky, in cold water and near-total darkness. This is not a thrill ride bolted into the rock — it is a committing wilderness descent. It is strictly for experienced canyoneers in good physical shape: a full seven-hour day of abseils up to around 22 metres, complex rope-work, sustained cold and no easy exit once you commit. Reputable operators insist on assessing your experience before they take you, and the minimum age is 18. If you have the background, it is a genuine once-in-a-lifetime line; if you don't, do Coanegra first and come back for this. Practical: From €95 per person · Expert / "very sporty" · ~7 hours · Prior canyoning experience required · Min age 18 · All technical kit included · Guided only.