Tools · Pascal's law

How hard the water pushes

At the surface you already carry one atmosphere — the whole column of air overhead. Water is about 800 times denser, so it stacks up fast: every 10 metres of depth adds one more atmosphere. The pressure you actually feel is the sum of the two — the air on top, plus one atmosphere for every ten metres of water:

That pressure pushes on everything, and the force adds up frighteningly fast: force is just pressure times area. Yet you don't get crushed, and that's Pascal's law at work — in a fluid, pressure presses equally in every direction at once. Your body is mostly water, which barely compresses, and your air spaces equalize to match, so the squeeze cancels itself out — the same squeeze that compresses a wetsuit and steals buoyancy as you descend. The only places you feel it are air pockets that haven't equalized. Pick a depth and a surface and watch the numbers climb.

Two things fall out of that little equation. First, the numbers get large quickly: at 30 metres you're under four atmospheres — share that among the gases in the mix and you get their partial pressures — about 59 psi, and a dive mask with that pressure difference across it would have hundreds of pounds trying to press it onto your face — which is exactly why you breathe out through your nose to equalize it. Second, and stranger: a palm-sized patch of you carries that same load, hundreds of pounds, and you feel nothing at all. That's the gift of Pascal's law and an incompressible body — the force is real, but it's matched on every side, so there's nothing left over to crush you.

It's the same reason a deep-diving fish isn't squashed and a submarine has to be built like a pressure vessel: what does damage is never the pressure itself, but a difference in pressure across something rigid or air-filled. Keep your air spaces equal to the water and the ocean's enormous push simply passes through you.

Your lungs are the biggest air space of all, which is why scuba works the way it does. Your breathing muscles are weak pumps — they can pull only about a tenth of an atmosphere — so on their own they could never expand your chest against the water at depth. That's exactly why a snorkel fails past about a metre down: the water squeezing your chest already outmatches what your diaphragm can draw. A regulator answers this by handing you gas at precisely the surrounding pressure, on demand, so the air in your lungs always matches the water outside. With no pressure difference left to fight, breathing at 30 metres feels much like breathing at the surface — a little more effort, since the gas is denser down there, but nothing like battling the whole water column.