The Crystal Cave of Giants was accidentally discovered in 2000 by miners working in the silver and lead mine at Naica, Mexico. It lies almost 300 meters (900 feet) below the surface of the Earth and it contains the largest crystals known in the world, by far. The largest crystals are over 11 meters long (36 feet) and weigh 55 tons.
The crystals themselves are made of selenite which is crystallized gypsum, the same material used in drywall construction. Except these crystals formed over a span of about half a million years in a hot water solution, saturated with minerals. The the temperature inside the cave remained very consistently hot for the entire time the crystals were growing.
It is still incredibly hot in the cave due its proximity to a magma chamber, deep underground. The air temperature is 50C with a relative humidity of over 90%, making the air feel like an unbearable 105C (228F) Entering the cave without special protective suits can be fatal in 15 minutes. I will be entering the cave wearing a special cooling suit with chilling packs inside and a specialized backpack respirator which will allow me to breath chilled air. Even with all this equipment, I will still only be able to stay in the cave for no more than 45 minutes at a time.
In extreme heat, the body begins to lose higher brain functions which made the expedition much more difficult with the risk of falling into deep pits, or being impaled on a sharp crystal. All the camera gear needs to be slowly brought up to temperature beforehand by pre-heating it and most cameras with moving parts and tape mechanisms simply will not work at all.
It is as dangerous as it is beautiful.