The longest underwater gypsum cave in the world — over five kilometres of flooded galleries under the left bank at Kungur. The water is clear enough for fifty metres of visibility, and divers describe flying rather than diving. The cave has killed several experienced divers: the sumps offer no landmarks, and someone who loses the line does not find the way back. Locals kept clear of the sinkholes long before anyone arrived with tanks.

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Blood Falls

A vividly red flow seeping from Antarctica's Taylor Glacier. The colour comes from iron in an ancient brine lake, sealed under the ice for millions of years along with microbes living without light.