Yes, there is such a thing as a geothermal lake and it’s just as incredible as you’d imagine. Lake Hévíz is Europe’s largest thermal lake and one of the largest swimmable thermal lakes in the world. It is also the ultimate rehabilitation destination, and home to Spa Hévíz and the Szent András Hospital. Because the lake is fed from both cold and hot underground springs, the temperature evens out at a perfect 90°F, so you could really spend all day floating around amongst the water lilies or sinking up to your thighs in the therapeutic peaty mud along the lake’s edges. Spa Hévíz opens the lake to visitors and retains all of its 18th century charm throughout its thoroughly modernized facilities including nine indoor pools.