A door label for thermonics on Regula I.
Thermonics was a geophysical method for groundwater exploration. This method was studied aboard Regula I in the late 23rd century.
In 2285, Drs. Carol and David Marcus walked out of the door labeled "Thermonics" and "Module 30" as they discussed Project Genesis.
Later Dr. Leonard McCoy walked into the thermonics section of Regula I, shortly before he discovered numerous scientists in the lab, all murdered by Khan Noonien Singh. (Star Trek II: The Wrath of Khan)