In 1960...

...these two guys, Jacques Piccard and Don Walsh, dove to Challenger Deep, the deepest part of the ocean, in a submarine (bathyscaphe). They were the first to do it and noone has ever done it again.

When they hit 9000 metres, one of the outside windows cracked due to the pressure, and they still had 1900 metres to go. Can you imagine how intensely scary that would be? The window cracking, and just the whole dive itself?

I'm surprised they were able to make a submarine big enough to fit both pairs of their balls.