Hydrostatic pressure
The pressure of a liquid's own weight, rising steadily with depth: about one extra atmosphere for every 10 metres of sea.
Hydrostatic pressure is the pressure a still liquid exerts because of the weight of the liquid above. The deeper you go, the more water is stacked above you, so the pressure rises in step with depth. It pushes equally in all directions, sideways and upwards as well as down, which is why it squeezes a submarine from every side.
At sea level the air already presses on us with about 14.7 pounds per square inch, one atmosphere. In the sea, every 10 metres (33 feet) of depth adds roughly another atmosphere. At 100 metres the total is about 11 atmospheres; at 4,000 metres, around 400; at the bottom of the deepest trenches, nearly 11 kilometres down, well over a thousand.
Only depth matters, not how much water there is. The pressure 10 metres down is the same in a narrow well as in the open ocean.
Liquids barely compress, so water-filled things cope well with depth. Deep-sea animals have no air spaces to crush and are mostly water. Deep-diving craft have used floats filled with a liquid lighter than water, such as petrol, rather than air, so they keep their buoyancy instead of being crushed. Gases, by contrast, shrink dramatically under pressure, as Boyle's law describes, and air-filled spaces need strong walls.
A common misconception is that pressure at depth only pushes downwards. It acts in every direction at once.
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