Which cave in Mexico contains the world's largest known crystals?

The Mexican cave containing the world's largest known crystals is the Cave of the Crystals, also called the Giant Crystal Cave. It is connected to the Naica Mine near Naica in Chihuahua, Mexico.

The chamber lies about 300 metres underground and contains giant selenite crystals, a transparent form of gypsum. The largest crystal measures approximately 11.4 metres long, has a volume of about 5 cubic metres and weighs an estimated 12 tonnes.

The crystals formed extraordinarily slowly over at least 500,000 years. Heated, mineral-rich groundwater from below interacted with cooler oxygenated water, creating conditions in which gypsum could crystallize at an exceptional scale. When the mine pumps are switched off, the chamber floods again.

The cave is dangerously hot and humid when dry: temperatures can reach about 58°C, with humidity between 90% and 99%. It is not normally open to tourists. The nearby Cave of Swords, discovered in 1910, also contains striking crystals, but its crystals are much smaller and it is a separate chamber.