There are tons of underwater mountains that have not even been mapped yet--and they are truly enormous

Seamounts are mountains rising from the ocean floor that don't reach the water's surface. They are therefore not islands. They were created by extinct volcanoes that rose abruptly, and It's been estimated that there are about 100,000 seamounts across the globe. They're usually 3,300–13,100 ft high, but because their peaks are hundreds to thousands of meters below the surface, many of them haven't been studied yet.

Because there are so many of them, large numbers of these underwater mountains have not even been mapped yet, and technology like bathymetry and satellite altimetry is being used to lessen the number of unmapped seamounts.

They are one of the most common oceanic ecosystems in the world and are home to plankton, corals, fish and marine mammals. This fact has been noted by the commercial fishing industry and extensive fishing is being done around many of them.

Supporting extensive fisheries has had a negative effect on these ecosystems and the decline of certain fish species is well documented. Bottom trawling is the biggest culprit and is the cause of 95% of the ecological damage. This method of fishing scrapes whole ecosystems off seamounts.

Over time, flanks of seamounts can collapse, causing sizable oceanic landslides. This can potentially cause huge tsunamis.

(Source)