swim upstream

basically when you want to get your advanced scuba certification (allowing you to go more than 60 feet deep) you have to undergo a very specific test: your instructor takes you down past the 60+ foot threshold, and she brings a little underwater white board with her.

she writes a very basic math problem on that board. 6 + 15. she shows it to you, and you have to solve it.

if you can solve it, you're good. that is the hardest part of the test.

because here's what happens: there is a subset of people, and we have no real idea why this happens only to them, who lose their minds at depth. they're not dying, they're not running out of oxygen, they just completely lose their sense of identity when deep in the sea.

a woman on a dive my instructor led once vanished during the course of the excursion. they were diving near this dropoff point, beyond which the depth exceeded 60 feet and he'd told them not to go down that way. the instructor made his way over to look for her and found a guy sitting at the edge of the dropoff (an underwater cliff situation) just staring down into the dark. the guy is okay, but he's at the threshold, spacing out, and mentally difficult to reach. they try to communicate, and finally the guy just points down into the dark, knowing he can't go down there, but he saw the woman go.

instructor is deep water certified and he goes down. he shines his light into the dark, down onto the seafloor which is at 90 feet below the surface. he sees the woman, her arms locked to her sides, moving like a fish, swimming furiously in circles in the pitch black.

she is hard to catch but he stops her and checks her remaining oxygen: she is almost out, on account of swimming a marathon for absolutely no reason. he is able to drag her back up, get her to a stable depth to decompress, and bring her to the surface safely.