"Subsurface temperatures" refer to the temperature conditions and changes that occur below the Earth's surface, specifically in the underground layers of soil, rocks, or water. Full definition
Teng, Haiyan and Grant Branstator, 2010: Initial - value Predictability of Prominent Modes of North Pacific Subsurface Temperature in a CGCM, Climate Dynamics, revised. (usclivar.org)
a, b, LOVECLIM and Trace21k (CCSM3) Southern Ocean subsurface temperature anomalies as time series (a) and in frequency space (b). (nature.com)
The new Cassini data reveal that temperatures at the polar fissures are more than 200 degrees Fahrenheit warmer than the rest of the moon, and Matson's analyses suggest that subsurface temperatures there are high enough to support chemical reactions among hydrocarbons. (discovermagazine.com)