Mexico's Cave of Crystals stunned geologists when it was first discovered in 2000.

The underground chamber contains some of the largest natural crystals ever found - some of the selenite structures have grown to more than 10m long.

Professor Iain Stewart got a rare glimpse of the subterranean spectacle while filming for the new BBC series How the Earth Made Us.

How the Earth Made Us: The epic story of how geology, geography and climate have influenced mankind is on Tuesday 19th January on BBC Two at 2100 GMT