On the north side of the factory area on the north edge of Elektrozavodsk ther is a concrete fence with that crack you can crawl under. I attempted to crawl from the inside to the outside and suddenly I was swimming in the water that is apparently present under the entire map.

In a desperate attempt to get out, I swam to the "actual water". Reaching the shore line I suddenly almost came to the surface, but no. It seemed I was still a pixel below the ground. And so i "swam" under ground to the bottom of the sea. For some reason I swam but the game did not really think I was under "proper water" because at a considerable depth I finaly died from hunger/exaustion/cuthulu, not the lack of air. Apparently I'm now a land shark and can breath dirt. Could come in handy some day.