The Blue Lagoon (Icelandic: "Bláa Lónið") geothermal spa is one of the most visited attractions in Iceland. The steamy waters are part of an other-worldly lava formation. The Blue Lagoon spa and geothermal complex is clearly visible from any of the usual satellite imagery sources at coordinates (63.879, -22.434).

The warm waters are rich in minerals like silica and sulfur and bathing in the Blue Lagoon is reputed to help many with skin diseases like psoriasis. Water temperature in the bathing and swimming area of the lagoon averages 40 °C (104 °F).

The lagoon is fed by the water output of a nearby geothermal power plant. Superheated water is vented from the ground near a lava flow and used to run turbines that generate electricity. After going through the turbines, the steam and hot water passes through a heat exchanger to provide heat for a municipal hot water heating system. Then the water is fed into the lagoon for recreational and medicinal users to bathe in.