Chapter 1 · From the surface to the Mariana Trench, 0–11,000 m
Deep under the water
Imagine you dive
Imagine you stand on the edge of a pool, then step into the sea and start to sink. Before you even get wet, something is already pressing on you. The air above your head weighs about 1 kilogram on every square centimetre of your body. We call this pressure 1 atmosphere, or 1 atm, and we do not feel it because the inside of our body pushes back with the same force. Now look at the picture beside the text. The diver hangs just under the surface, and the first number of the sandbox says 1 atm. Drag the depth slider a little and watch that number start to climb.
Ten metres, one more atmosphere
Water is heavy. A column of sea water 10 metres tall weighs about as much as the whole column of air above it, so every 10 metres of depth adds about 1 atm. The rule is pressure = 1 + depth / 10, with depth in metres. Worked example at 30 m: 1 + 30 / 10 = 4 atm. One atmosphere is the air and three more are the water. On every square centimetre of your skin there is now about 4 kilograms. Move the slider to 30 m in the sandbox and check the result yourself.
Why a balloon shrinks
Gas can be squeezed, and the rule is called Boyle's law: when the pressure doubles, the volume halves. A balloon that holds one litre at the surface holds half a litre at 10 m, where the pressure is 2 atm, and a quarter of a litre at 30 m, where it is 4 atm. Your ears, sinuses and mask are air spaces too, so a diver gently equalises them on the way down. The same law has a warning on the way up: a scuba diver must never hold the breath while rising, because the air in the lungs expands and can over-inflate them.
The sea loses its colours
Light does not travel far in water. Red is absorbed first and fades within the first 5 to 10 metres, then orange and yellow disappear, and blue goes deepest. That is why the sea looks blue and why a red fish looks grey at depth. Below about 200 metres begins the twilight zone, where only a faint blue glow is left. Below about 1,000 metres there is no sunlight at all, in the midnight zone. The deep sea is also cold, usually only 2 to 4 degrees Celsius. Slide the depth and watch the colour of the water change.
The limits of the body
The body is built for air at 1 atm, and it protests when the pressure grows. Recreational scuba diving usually stays within 30 to 40 metres. From around 30 m many people feel nitrogen narcosis, a dreamy confusion like being tipsy. Under pressure nitrogen from the breathing gas dissolves in the blood; if a diver rises too fast, it forms bubbles, and this is decompression sickness, called the bends. That is why divers rise slowly and stop on the way up. Diving is learned only with a qualified instructor, never from a book or a video.
One breath, 214 metres
A free diver has no tank and goes down on a single breath. In 2007 the Austrian Herbert Nitsch reached 214 metres in the no-limits discipline, a record in the no-limits discipline, where a weighted sled takes the diver down. At that depth the pressure was about 1 + 214 / 10 = 22.4 atm, more than twenty times what we feel at the surface. Such dives are extremely dangerous even for champions. Animals do it every day: a sperm whale hunts at about 2,000 metres and holds its breath for an hour or so.
The bottom of the world
The deepest known place in the ocean is Challenger Deep in the Mariana Trench, about 10,900 to 11,000 metres down, and the figure often quoted is 10,935 metres. Pressure there is about 1 + 10,935 / 10, or roughly 1,100 atm, which is about a tonne on every square centimetre. Two men first reached it in the bathyscaphe Trieste in 1960: Jacques Piccard and Don Walsh. In 2012 the filmmaker James Cameron dived there alone. Even here there is life: a snailfish lives at about 8,000 metres. Move the slider to the very end to see the bottom.
In numbers
- +1 atm
- added by every 10 m of sea water
- 4 atm
- the pressure at 30 m: 1 + 30 / 10
- ≈ 1,100 atm
- at Challenger Deep, about 10,935 m