The sea presses in from every side. Each ten metres you sink adds roughly one atmosphere — so at full-ocean depth, near 11 000 m, a hull carries over a thousand atmospheres, about a tonne pushing on every square centimetre. That is why crewed deep-sea craft are “humans in a can.”
Shape decides everything. A sphere shares the squeeze evenly across its whole shell, so its wall carries only half the stress of a cylinder’s and it resists collapse far better. A cylinder wants to be squashed into an oval — it fails by buckling, and its crush pressure scales as the cube of the wall ratio (t/R)³, so halving the wall drops the crush depth eightfold.
Thickness buys depth, but at a brutal price: the hull’s mass climbs in step with every millimetre you add, and mass must be carried down and hauled back up. The record holders answer with a thick titanium sphere — strong, comparatively light, and the only shape that shares the load.
In 2023 the Titan submersible imploded on the way to the Titanic. Its hull was a carbon-fibre cylinder — the wrong shape, in a material that fatigues under repeated deep dives. It was, at heart, a shape-and-materials problem. Drive one down here and watch it happen. (Depths and gauges are computed from real formulae; the visible compression is exaggerated so you can see it.)
Something in the simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.