Which is the deepest ocean trench?

The Mariana Trench, whose deepest point, Challenger Deep, is just under 11,000 metres. Everest would fit inside with a couple of kilometres of water on top. The pressure at the bottom is over a thousand atmospheres - roughly a tonne on every square centimetre.

Three crewed descents have been made in sixty years. Jacques Piccard and Don Walsh went down in 1960 in the bathyscaphe Trieste, stirred up so much silt they could see almost nothing, and stayed twenty minutes. James Cameron went alone in 2012. Victor Vescovo made repeated dives from 2019 in a submersible designed to do it routinely, and found a plastic bag.

There is life all the way down - amphipods, sea cucumbers, single-celled organisms in the sediment - and it is not sparse. What surprised researchers is that the trench is not a barren extreme so much as a collection point: a funnel that gathers whatever sinks, which is why plastic fragments and synthetic fibres turn up in animals at the bottom in higher proportions than in shallower water.