The search and rescue mission for OceanGate’s Titan, a five-person submersible that disappeared en route to the Titanic shipwreck on Sunday, June 18, has generated lots of questions. One of them involves terminology: Why is everyone calling the vessel a submersible, rather than a submarine?

In short, because the Titan is a submersible, not a submarine. The two words aren’t direct synonyms. Here’s how oceanographer Edith Widder explained the main distinction in a video for the National Oceanic and Atmospheric Administration:

“The difference between a submarine and a submersible is [that] a submarine has enough power to leave port and come back to port under its own power. A submersible has very limited power reserves, so it needs a mother ship that can launch it and recover it.”

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There are various kinds of submersibles. Some are underwater drones of sorts, either remotely operated in real time or pre-programmed to traverse a certain route. Others, like the Titan, harbor human crews. But whether or not people are on board, submersibles—relative to submarines—are pretty small and simply constructed, and only equipped for short-term dives.

The Titan, which is roughly the size of a minivan, is meant to stay submerged for around 10 hours and has a four-day supply of oxygen for its five passengers. Some submarines, by contrast, are longer than a football field, can generate their own oxygen, and might stay submerged for a few months at a time.

Linguistically speaking, submersible can also be an adjective that essentially means “able to be submerged,” and submarine can describe anything below the surface of the sea. But if you’re specifically talking about types of vessels, you can’t accurately use the terms interchangeably.

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