These milky turquoise waters may look inviting for a swim, but you wouldn't want to take a dip here. This caldera, formed in the crater of the Kawah Ijen volcano, is the world’s largest acidic lake.

The electric blue flames that burst into the air are a source of fascination, although (of course) there is a scientific explanation. High levels of sulphuric acid, which give the water its striking hue, combust when they hit the air.