Underwater gliders have become an increasingly valuable tool for oceanographic research. Ditching traditional propellers and thrusters, they move about by means of variable buoyancy propulsion, which is a cumbersome way of saying that they propel themselves by rising in the water and then using hydrofoils to control their direction as they descend.

It's not very fast, but it is economical and allows the gliders to carry out long missions across thousands of miles to monitor ocean conditions, seek out pollution, and conduct military reconnaissance as they dive to depths of up to 1,000 m (3,300 ft). They are also much cheaper to operate than research vessels, so what's the problem?