Ocean-going fish can’t live any deeper than 8200 meters, according to a new study. All fish have their limits—you’ll never find sharks below 4 kilometers, for example—but why there aren’t any fish at all below 8 kilometers remains a mystery. Now, a team of biologists say the threshold is set by two competing effects of trimethylamine N-oxide (TMAO), a chemical in fish cells that prevents proteins from collapsing under high pressure. While fish should need more and more TMAO to survive ever greater depths, higher concentrations of the compound also draw in more and more seawater through osmosis, the process by which cells regulate their water content. In the deepest waters, high TMAO levels reverse osmosis pressure, swelling brain cells to the point that they stop working and, in principle, bursting red blood cells open. (The team says they’re still working on how other marine creatures like anemones and bacteria avoid such gruesome fates at the most extreme depths, but they suspect those organisms produce more efficient protein boosters than fish can.) To test that claim, the team looked 7000 meters down in the Kermadec Trench north of New Zealand. There, they captured five Notoliparis kermadecensis snailfish (pictured above alongside a brittle star, Ophiura loveni), whose record TMAO levels and osmosis pressures matched projections the researchers made based on shallower dwelling fish. Extrapolating the new results just a bit further, they find osmosis should reverse itself at a depth of 8200 meters—right about where fish no longer swim the sea.