Blue Lagoon: the waters of this gorgeous geothermal spa are rich in blue-green algae (hence the colour), mineral salts and fine silica. The lagoon is surrounded by a very surreal landscape of black lava, and the water temperature of 37-39°C ensures that uninterrupted (and rahter mysterious) steam blows across the surface. The whole thing makes you think you are an extra on a sci-fi movie set!