The Challenger Deep

It is part of the Marianna trench, it's called Challenger Deep. Two American naval officers went there on a Bathysphere in the 1950s. So what you do is you would have to use explosive ranging to get that deep. You set up some hydrophones, which are just microphones underwater, and you would then set off dynamite. And measure the echo? Yeah, the pressure wave would then be enough to get all the way down and back up.