The Blue Lagoon geothermal spa is one of the most visited attractions in Iceland. The spa is located in a lava field in Grindavík on the Reykjanes Peninsula, southwestern Iceland. It is a man made lagoon which is fed by the water output of the nearby geothermal power plant Svartsengi and is renewed every two days.

The warm waters are rich in minerals like silica and sulfur and bathing in the Blue Lagoon is reputed to help some people suffering from skin diseases such as psoriasis. The water temperature in the bathing and swimming area of the lagoon averages 37–39 °C (99–102 °F). The Blue Lagoon also operates a research and development facility to help find cures for other skin ailments using the mineral rich water.

More Info: en.wikipedia.org