The submersible sinks or rises at the chosen descent rate. Hydrostatic pressure grows almost perfectly linearly with depth — roughly one extra atmosphere every 10 metres of seawater — while sunlight fades exponentially, because every metre of water absorbs and scatters a fixed fraction of the light passing through it. The two curves shape everything about deep-sea life and engineering: pressure dictates hull design, light (or its absence) dictates who can photosynthesise, hunt by sight, or must rely on bioluminescence and chemosynthesis instead.
pressure(d) = 1 atm + d / 10 [≈1 atm per 10 m]
light(d) = 100% · e^(−d / 165) [clear-water attenuation]
hull stress = pressure(d) / pressure(rating)
- Descent rate — vertical speed of the submersible in metres per second; real deep-diving craft descend at roughly 0.3–1 m/s to manage ballast and thermal stress.
- Hull rating — the crush depth the pressure hull is engineered for. Passing it in the simulation flashes a hull-breach warning, the same margin real submersibles are certified never to exceed.
- Headlights — below the twilight zone almost no sunlight survives, so external floodlights become the only way to see the hull's surroundings; bioluminescent organisms are the only other light source.
The five bands you pass through mirror real oceanography: the sunlit epipelagic (0–200 m) supports photosynthesis; the twilight mesopelagic (200–1,000 m) has just enough light for silhouette-hunting predators; below that, the bathypelagic, abyssopelagic and hadal zones are in permanent darkness, populated mostly by bioluminescent and chemosynthetic life clustered around hydrothermal vents on the seafloor, visible here near the Challenger Deep.