The spectacular but dangerous cave of crystals

In the year 2000, two brothers were working in the Naica lead and silver mines when they came across a chamber quite by chance. They found it packed with some of the biggest natural crystals on Earth: huge translucent beams and obelisks of selenite or moonstone, a form of gypsum. The largest were 36ft long and more than half a million years old. It was a spectacular discovery.

The Cave of Crystals lies 1,000ft below ground level in the Naica mines near the town of Delicias in Chihuahua State, northern Mexico. They are working mines, so access to the Cave of Crystals is restricted. Visits are possible to the Cave of Swords, which contains many stunning, sword-sized crystals.

This extraordinarily beautiful place is also perilous. Molten magma deep below the cave keeps the temperature constantly high at around 50C. This, together with 90 to 100 per cent relative humidity, is potentially lethal. To avoid heat stroke, visitors to the cave must wear suits packed with ice cubes and face masks blowing ice-cooled air.

Yet it is this heat that is key to the crystal formation. For millennia, the chamber was flooded with mineral-rich water, and the hot, stable conditions were ideal for crystals to form. When, in 1985, water was pumped from the mines, the crystal formation ceased. There is now a danger that, having been exposed to the air, the giant crystals will slowly collapse, and there are calls to preserve the site.

The walls of the Lascaux Caves in southwestern France are decorated with Palaeolithic rock art more than 17,000 years old. The images are of large animals, including a 17ft long aurochs (the ancestor of domestic cattle), as well as horses and deer. Today, access to the caves is restricted to scientific experts, but a replica of the two main halls of the cave, known as Lascaux II, is open to visitors.

Deep under the Gabon rainforest in west central Africa, the Abanda Caves are host to an enormous roost of up to 100,000 bats of different species. But while looking up, visitors must also watch their steps below, because a small population of orange crocodiles lives on the floor of the caves.