A subsurface ocean refers to a large body of water that exists below the surface of a planet or moon, hidden beneath its outer layers such as ice or rock. Full definition
Extraterrestrial Life May Be Hiding in Subsurface Oceans of Distant Planets (newsweek.com)
«For example, we found that the increase in subsurface ocean temperatures preceded the observed warming of surface air and sea surface temperatures, which began in the 1970's.» (noaanews.noaa.gov)
Contributions of subsurface ocean temperature data have come from all countries that make oceanographic measurements. (noaanews.noaa.gov)