Lake Hévíz, Hungary

Swimmers float out from the bathhouse on stilts into Lake Hévíz in western Hungary, generally described as the largest swimmable thermal lake in the world at about 4.6 hectares (11 acres). A spring at the bottom of the lake keeps it warm all year, at roughly 33 to 36°C (91 to 97°F) in summer and about 24 to 28°C (75 to 82°F) in winter, so a cold-weather swim is part of the appeal.