A hot spring was a spring produced by the emergence of geothermally heated groundwater from a planet's crust.
There was at least one underground hot spring on the planet Sarpeidon. In 2269, Zarabeth mentioned this to Spock when he asked her what the source of heat in her shelter was. (TOS: "All Our Yesterdays")
During a trip to Risa in 2373, Quark suggested that he, Julian Bashir and Jadzia Dax should go skinny dipping in the Risian hot springs. (DS9: "Let He Who Is Without Sin...")
In 2376, Tuvok said it was theoretically possible for L class planetoids to contain hot water springs that could sustain a stranded individual for several weeks. (VOY: "Riddles")