Undersea means existing, happening, or operating beneath the surface of the ocean. An undersea cable carries internet signals along the ocean floor, connecting continents so you can video chat with someone across the world. An undersea volcano erupts on the seafloor, sometimes creating new islands. An undersea earthquake can trigger a tsunami.

The word describes a vast, mysterious world that covers most of our planet. Scientists use undersea robots and submarines to explore depths where the pressure would crush a human. They've discovered undersea mountain ranges taller than the Himalayas, hidden canyons deeper than the Grand Canyon, and strange creatures that glow in the darkness.

People also use undersea to describe human activities in the ocean: undersea exploration, undersea mining, or undersea archaeology. When treasure hunters search for sunken Spanish galleons, they're doing undersea archaeology. The word emphasizes that something is specifically under the sea, not just in or near it.

You might also see underwater, which means almost the same thing but works more broadly. Undersea specifically refers to the ocean environment, while underwater can describe anything submerged, even a flooded basement.

As an adverb, undersea means “beneath the sea,” as in “The cable runs undersea.”