Until you notice the orange-suited men clambering around, it's hard to grasp the extraordinary scale of this underground crystal forest.
Nearly 1,000ft below the Chihuahua Desert in Mexico, this cave was discovered by two brothers drilling in the Naica lead and silver mine. It is an eerie sight. Up to 170 giant, luminous obelisks - the biggest is 37.4ft long and the equivalent height of six men - jut across the grotto like tangled pillars of light; and the damp rock of their walls is covered with yet more flawless clusters of blade-sharp crystal.
They are formed from groundwater saturated in calcium sulphate which, warmed by an intrusion of magma about a mile below, began filtering through the cave system millions of years ago. When, about 600,000 years ago, the magma began to cool, the minerals started to precipitate out of the water, and over the centuries the tiny crystals they formed grew and grew until 1985, when miners unwittingly drained the cave as they lowered the water table with mine pumps.