Volcanic spring

A volcanic spring was a spring of hot water, often found underground, heated by a planet's volcanic activity. On Earth, volcanic springs could be found in Iceland. Volcanic springs could also be found below an ice cave on the planet Marinus. (TV: The Keys of Marinus)

Following the great earthquake of 62 AD, an innovation from the soothsayers led the Roman town of Pompeii to use hypocausts with hot springs leading from Mount Vesuvius. (TV: The Fires of Pompeii)