What Lies in the Multiverse

You're naturally buoyant, so you can't simply swim down and stay there waiting for you air to run out, you have to find a pool of water with a top-right or top-left corner underwater, where the water touches a ceiling. To get down far enough, you'll probably have to jump into the water and swim under the corner before you surface. Once you get there though, just wait a few seconds and you'll see a blue air gauge begin to tick down. Wait for it to completely drain, and the achievement will pop.

Faily poor ascii art, but this should give you an idea of the type of area to look for:

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