Bottom Crawlers traverse the seabed; the Benthic Rover (pictured left) developed by the Monterey Bay Aquarium Research Institute (MBARI) is currently one of the only Autonomous Bottom Crawlers. 

travelling along the water's surface, ASVs often draw energy from their environment (e.g. via the sun, wind, or waves). One of their main advantages is the ability to constantly transmit/receive positioning information and data.

Able to execute missions without a human operator, AUVs are untethered (unlike ROVs) and have active propulsion - e.g. motors and thrusters (unlike  gliders).

Ocean gliders rely on passive propulsion, using changes in buoyancy and ocean currents. Although autonomous, they are generally incapable of decision-making. See GLIDERS page for more information.

Considered a UUV, HROVs can act as either an AUV or an ROV. An example of an HROV is the Nereus [2], which was developed by the world-renowned Woods Hole Oceanographic Institution (WHOI).

I-AUVs herald the new age of underwater robots that are able to both process information and act on it. I-AUVs have demonstrated abilities such as turning valves and pushing buttons - will they one day replace ROVs?

Requiring a tether, or "umbilical" that connects them to a pilot and also provides power, an ROV is technically manned as it cannot work without a human operator.

Invented in 1981 by International Submarine Engineering (ISE) [5], the body of a semi-submersible AUV is fully underwater. It has a surface-piercing mast that provides air for a diesel engine and the ability to use GPS.

The term used by the naval/military community for (Autonomous) Bottom Crawler. I prefer to avoid this term as the military also uses this acronym for their above-water counterparts, 'Unmanned Ground Vehicles'.

The term used by the naval/military community for ASV. Again, I prefer to avoid the military term as the use of the letter 'U' for both 'Unmanned' and 'Underwater' can be confusing.

While many use the terms 'UUV' and 'AUV' interchangeably, they are different. 'UUV' is a term coined by the US Navy in the 1990s and refers to both AUVs and ROVs.