Question: How come the water pressure deep down in the sea is possible to crush humans, but isn't able to kill fish?

Hayley Evers-King answered on 12 Mar 2013:

There are a couple of reasons for this. Humans have a lot of air in them and air is compressible under pressure. Deep sea fish don’t have a lot of air in them, even compared to fish that live higher up in the water column, they often don’t have swim bladders and have different skeletal structures. Deep sea fish are also often quite small, this minimises the effects of pressure. They are very well adapted to this high pressure environment, in fact if you bring them to the surface, they don’t do very well at all! This has been a big problem for marine biologists who have had to design specialised traps.