Nevertheless, whilst conditions in the cave are ideal for crystals, they are deadly to humans. Humidity stands at more than 90 per cent – a level at which sweating has no cooling effect on the body.

Discovered in 1999 inside an abandoned mine in Southern Spain, Pulpí Geode is the largest crystal 'cave' of its kind in the world. But what exactly is a geode and how do crystals form inside them?