Richard Ha writes:

Q: How deep is the Blue Lagoon geothermal spa in Iceland?

From Wikipedia: The warm waters are rich in minerals like silica and sulphur and bathing in the Blue Lagoon is reputed to help some people suffering from skin diseases such as psoriasis.[1] The water temperature in the bathing and swimming area of the lagoon averages 37–39 °C (98–102 °F). The Blue Lagoon also operates a Research and Development facility to help find cures for other skin ailments using the mineral-rich water.

The lagoon is a man-made lagoon which is fed by the water output of the nearby geothermal power plant Svartsengi and is renewed every 2 days. 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 water heating system. Then the water is fed into the lagoon for recreational and medicinal users to bathe in.

A: I was there, and it’s waist high. People don’t stand up because it’s too cold outside. The steam rising in the back is partly H2S. But as with Japan’s onsens, the Blue Lagoon is looked upon as providing a health benefit.

More pictures of the Blue Lagoon here.