Seljavallalaug Swimming Pool in Iceland is the oldest pool in the country, built in 1923 to teach people how to swim. The pool is fed by geothermal hot water, but don't expect it to be super hot - more like a heated swimming pool. The hike to get there takes about 20 minutes each way, and the changing rooms are pretty gross due to tourists leaving behind trash and wet clothing. The water is filled with algae, so maybe skip putting your head under. It's free to swim there, but remember to leave no trace and help keep this hidden gem clean for everyone to enjoy!