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 natural hot water, but don't expect a hot tub experience - the temperature ranges from 68 to 86°F. The 20-minute hike to the pool offers scenic views, but be prepared for the disgusting changing rooms filled with trash and algae-filled water. Despite its issues, entrance is free, so bring a plastic bag for your waste and help keep this hidden gem clean for others to enjoy.