Caves can be exciting to explore - but not when they're a threat to human life.
One such cave in Chihuahua, Mexico is so unique but holds a deadly secret.
The underground cavern, lying 300 metres (980 feet) beneath the Earth, is filled with some of the world's largest natural crystals, some as large as trees.
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Interestingly, it was discovered by accident in 2000 when miners drilling into a lead, zinc, and silver mine near the town of Naica uncovered a chamber full of towering, milky-white crystals.
'It's the Sistine Chapel of crystals,' stated Juan Manuel García-Ruiz, a geologist with the Spanish National Research Council (CSIC), and a researcher who studied the cave.
Fault lines run beneath the Sierra de Naica Mountain, and about 26 million years ago, magma pushed toward the surface through these faults.
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This resulted in the mountain's formation along with the jumbo-sized crystals.
As beautiful as the crystals are, the scene can't be appreciated long in there as the same environment is deadly to humans.
The cave's temperature stands at 58 degrees Celsius (136 degrees Fahrenheit), sometimes going above that.
Moreover, the humidity is almost 100% – a level at which sweating has no cooling effect on the body.
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Because of this, people need proper protection to stay in the cave for longer than 10 minutes.
Without proper protection, staying longer than 10 minutes is dangerous, as fluid can condense in the lungs and prove fatal.
Hence why this is a no-tourist zone.
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Furthermore, the condensation makes surfaces extremely slippery, making the cave's crystals a dangerous expedition.
Now the fact that it's deadly to humans keeps the majority of us away, the good news is research can still be done in the cave by those who enjoy the wilder side of life.
For example, scientists are learning about undiscovered microorganisms that thrive in these extreme conditions.
'Other people have made longer-term claims for the antiquity of organisms that were still alive, but in this case, these organisms are all very extraordinary - they are not very closely related to anything in the known genetic databases,' said Dr. Penelope Boston, director of Nasa's Astrobiology Institute in Moffett Field, California.