Cueva de los Cristales (Cave of the Crystals) in Mexico is a cave 300 meter underground containing some of the largest natural crystals. To give you an idea of how big these crystals are, the largest one found to date is 11m long, 4m wide, and weight 55 tons.

The crystals are made of selenite, a type of gypsum, and are translucent. At first, geologists did not understand how cyrstals can grow to be so big, but they figured it out and published in the journal Geology. By studying tiny pockets of fluid trapped inside, the scientists figured that the crystals thrived because they were submerged in mineral-rich water with a very narrow, stable temperature range of around 54°C.

And now the BBC’s Professor Iain Stewart has gone exploring, producing amazing footage. Here is a preview of the show.

The cave was discovered by two miners excavating a new tunnel in the Naica mine in the Chihuahuan Desert in Mexico for the Industrias Peñoles company in 2000. More photos from National Geographic.