A small, usually human-occupied vehicle designed to dive deep beneath the ocean surface is a submersible. Unlike a submarine, a submersible typically relies on a support ship for launch, recovery, and recharging, and it operates for hours rather than weeks.

Thick pressure hulls, often spherical for strength, protect the occupants. Viewports, thrusters, manipulators, and scientific sensors allow observation and sampling. Famous submersibles such as Alvin have carried scientists to hydrothermal vents, deep trenches, and the wreck of the Titanic. Battery power, life support, and communication with the surface limit dive duration.

Human-occupied submersibles provide an unmatched presence for complex manipulation and real-time decision-making. They are complemented by remotely operated and autonomous vehicles that can stay down longer and go places too risky for people. The experience of looking out a viewport at an active black smoker or a never-before-seen animal continues to inspire both science and public interest in the deep ocean.

Only a few dozen human-occupied submersibles capable of reaching abyssal depths exist in the world.