"Undersea mountains" refer to large, elevated structures that are found underwater. These are like mountains that exist in the ocean depths, but covered by water instead of being above the surface. Full definition
That's because the gravitational attraction from a massive undersea mountain range causes water to pile up in a broad sea - surface bump several feet high. (sciencenewsforstudents.org)
So yes, Rapa Nui sits way out in the ass end of nowhere, atop a seamount that has formed via the Easter hotspot, an upwelling of magma below the oceanic crust that has generated a range of undersea mountains (the Nazca Ridge) as the Nazca Plate drifted above it.... (sydkab.com)
Whereas the nodules are scattered across the deep abyssal plains of the oceans, hundreds of miles from shore and typically three miles or more below the surface, many of the sulfide deposits are close to a coastline; also, they are always on undersea mountains and therefore located in much shallower water. (discovermagazine.com)