One of the most visited attractions in Iceland is the Blue Lagoon, or rather Blàa Lónið. Blue Lagoon is a geothermal spa located in a lava field in the southwest, not that far away from the international airport. The 37-39°C warm water is rich in minerals and good for people suffering skin diseases. The lagoon is not natural, it is water output from the nearby geothermal power plant. Considering it that way, you bath in expensive „residue/waste“ water.