To found a bridge pier on rock, you have to reach the rock. If the rock is under thirty metres of river, mud and sand, somebody has to work down there, in the dry, on the riverbed itself.

A pneumatic caisson is an upturned box, open at the bottom, sunk to the riverbed and filled with compressed air. The air pushes the water out and holds it out, and men dig inside a bubble held down by the weight of the structure being built on top. Material goes out and men go in and out through air locks. As the digging deepens, the box sinks under its own growing weight.

The pressure required is not a design choice. Water pushes in with a pressure of ρgh, so the air inside must match the pressure at the cutting edge — about one extra atmosphere for every ten metres of depth, and no amount of engineering changes that number.

Which is where the physics turns into physiology. Under pressure, Henry's law says more nitrogen dissolves into the blood and tissues of the men breathing it. Bring them up quickly and the dissolved gas comes out of solution inside them, as bubbles. On the Eads and Brooklyn bridges the men worked at pressures up to about 55 psi above atmospheric, and they were crippled and killed by it. Dr Andrew Smith, the Brooklyn Bridge physician, named it caisson disease in 1873. Washington Roebling, the chief engineer, went down in 1871 and was disabled for the rest of his life.

Sink a working caisson in a bucket, measure the pressure it needs, and open a bottle of fizzy water to see the rest.

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