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 hot - it's more like a heated swimming pool. The hike to the pool takes about 20 minutes each way, and the changing rooms are in terrible condition due to tourists leaving trash behind. Despite its beauty, the water is filled with algae and the temperature is lukewarm at best. It's free to swim at Seljavallalaug, but visitors are encouraged to leave no trace and help maintain this hidden gem for future enjoyment.