Submarines control depth by flooding or blowing ballast tanks to change buoyancy, then fine-tune trim with smaller tanks and dive planes as they descend. Sound travels roughly 4.3× faster in seawater (~1500 m/s) than in air (~343 m/s), which is why sonar — not radar or light — is the primary way submarines "see" underwater; active sonar emits a ping and listens for the echo, while passive sonar just listens. Water pressure increases by about one atmosphere every 10 meters of depth, so hulls must be built to resist immense crushing force, and sunlight fades quickly, with the photic zone ending near 200 m and the pitch-black "midnight zone" beginning below 1000 m.

• Sound travels about 1500 m/s in seawater versus ~343 m/s in air
• Water pressure rises roughly 1 atmosphere for every 10 m of depth
• Military submarines typically have a test depth near 300–500 m, with crush depth well beyond that
• Ballast tanks are flooded to dive and blown with compressed air to surface
• Controlled dives typically descend at just a few meters per minute
• The sunlit photic zone ends around 200 m; the "midnight zone" begins below 1000 m
• Active sonar pings reveal a target's range and bearing; passive sonar only listens