The deepest borehole in the world — 12,262 metres, drilled between 1970 and 1992. Science got surprises out of it: water at a depth where there should be none, and microscopic fossils in rock two billion years old. Urban legend added its own: that a microphone lowered into the hole recorded screaming, and drilling was abandoned because of a well to hell. No such recording exists and the project ended when funding collapsed, but the story lives on its own.

Read next

The Ordinskaya Cave

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.