Paul D Stewart / Aerial view of Grand Prismatic thermal spring in Yellowstone National Park, USA. The bright colours are natural, coming from thermophile bacteria in the water. People can be seen walking on the raised...

Jeff Vanuga / Quagmire Group thermal pools in Porcupine Hills Geyser Basin. Yellowstone National Park, Wyoming, August 2009.

Jeff Vanuga / Thermal pools and hot springs along White Creek in the Lower Geyser Baisn area of Yellowstone National Park, Wyoming. June 2010.

Jeff Vanuga / Thermal pools and hot springs along White Creek in the Lower Geyser Basin area of Yellowstone National Park, Wyoming. June 2010.

Inaki Relanzon / Bathers relaxing in The Blue Lagoon, Thermal area and swimming pools on the Reykjanes Peninsula, near Reykjavik, Iceland

Inaki Relanzon / Bathers relaxing in The Blue Lagoon, Thermal area and swimming pools on the Reykjanes Peninsula, near Reykjavik, Iceland

Steven David Miller / Boiling mud at Leaping Frog mudpool, Whakarewarewa Thermal reserve, Rotorua, North Island, New Zealand

Barry Bland / Blue Lagoon thermal pools spa, Iceland.

Tui De Roy / Geothermal activity, mineral hot pools, geyser thermal field, Iceland