undersea
adj.located or happening below the surface of the sea. You use this to describe things like cables, animals, or structures that are deep in the ocean.
adj.situated or occurring beneath the surface of the sea. Often used to describe infrastructure, marine life, or geological features found in the ocean depths.
“The divers explored the undersea world of coral reefs.”
“Engineers installed a high-speed undersea cable to connect the two islands for better internet service.”
“The documentary provides a fascinating look at the undersea ecosystem, revealing how life thrives in the extreme pressure and darkness of the deep ocean floor.”
“The agency located the shallow undersea quake near Flores island.”
“Separately, Australia’s tsunami warning centre said the undersea earthquake will have “no tsunami threat to the Australian mainland, islands or territories”.”
From under- + sea.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.