Why is the Cave of the Crystals or Giant Crystal Cave closed to the public?

Context: Cave of the Crystals or Giant Crystal Cave is a cave connected to the Naica Mine at a depth of 300 metres, in Naica, Chihuahua, Mexico. It takes the form of a chamber within the limestone host rock of the mine, and is about 109-metre long with a volume of 5,000 to 6,000 cubic metres.

The chamber contains giant selenite crystals, some of the largest natural crystals ever found. The largest is 11.40 metres, with a volume of about 5 cubic metres, and an estimated mass of 12 tonnes. When not flooded, the cave is extremely hot, with air temperatures reaching up to 58 °C with 90 to 99 percent humidity. This is comparable to temperature records in Death Valley, but with much wetter air. The cave is relatively unexplored because of these factors. Without proper protection, people can only endure approximately ten minutes of exposure at a time.

The cave was discovered in April 2000 by brothers Juan and Pedro Sánchez while drilling in the mine. As of October 2015, the mine had reflooded and the cavern filled once more with the water rich in minerals required for the crystals to grow.

A group of scientists in the Naica Project have been studying these caverns.

Answer: The conditions in the cave are not hospitable, extreme heat and humidity, sharp slippery crystals are lethal for humans.