> Other than hydrothermal vents, what natural heat sources are you thinking of? Mostly the sun, but also warm freshwater rivers (one of the biggest temperature

> Other than hydrothermal vents, what natural heat sources are you thinking of? Mostly the sun, but also warm freshwater rivers (one of the biggest temperature differentials you'll find excluding hydrothermal vents). Areas with a lot of organic material (algae etc.) close to the shore can also get very warm compared to the temperature average. Though I'm not sure about the cause/effect for the latter.