The sea gets heavier as you sink

To reach the bottom of the Challenger Deep in 1960, the bathyscaphe Trieste had to carry two crew in a steel sphere small enough to survive about 1,100 atmospheres. A sphere that thick and heavy sinks like a stone, so it hung under a huge float. That float wasn't filled with air. Any air-filled tank big enough to lift the sphere would have been crushed flat long before the bottom. It was filled with gasoline instead: lighter than water, liquid so it couldn't be crushed into nothing, and left open to the sea at the bottom so the pressure inside and outside always matched.

There was a catch. Gasoline compresses more than seawater does. As Trieste sank, her float slowly lost lift and she got heavier, accelerating toward the bottom. To control that, she carried tonnes of iron shot in hoppers, held shut by electromagnets. The pilots let shot go to slow the descent and to come back up. If the power failed, the magnets let go, the shot poured out and Trieste rose on her own. Gravity was the fail-safe.

Trieste kept working after the record dive. In 1963 she went down off New England to look for the nuclear submarine Thresher, lost with all 129 aboard, and photographed her wreckage at about 2,500 metres. Half a century later, James Cameron's Deepsea Challenger made the same solo trip to the Challenger Deep in a vehicle whose structure was largely syntactic foam: billions of tiny hollow glass spheres set in resin, a solid that floats and barely compresses at all.

Compressibility: buoyancy is not a constant

Pressure. Every 10 metres of seawater adds about one atmosphere. At 11,000 metres the water itself is squeezed denser by around 5%. That sounds small, but buoyancy is a difference between two big numbers, and a few percent of a tonne-scale vehicle is hundreds of kilograms.

Stiffness. Whatever you float on also compresses, at its own rate. A material that squeezes more than water, like gasoline, loses lift as you descend, and the deeper you go the faster you fall. A material stiffer than water, like good syntactic foam, gains a little lift. An air-filled hull hardly changes until its rated depth, and then it implodes. Designers match their flotation to the depth they're going to, and carry ballast they can drop to make up the difference.

Three answers. The Builder sorts every hull into three strategies. Resist: a dry, one-atmosphere housing, heavy and thick-walled, which is what the Trieste sphere was. Compensate: fill the volume with oil or gasoline so pressure inside matches outside, which is what the Trieste float did. Comply: make the whole body from soft material about as stiff as water, so depth almost stops mattering, the way a jellyfish does. Every deep vehicle mixes all three.

One vehicle, trimmed to hover at the surface, with three choices of float. Drag it down toward the Challenger Deep and watch the net buoyancy drift. Drop shot to stay in control, and find out what happens to the air tank.