1. Naica, Mexico – Cave of Crystals

2. Scientists rest in a cooling tent at an underground base camp outside the Cave of Crystals. The cavern's combination of 90 percent humidity and a temperature of 118 degrees Fahrenheit (48 degrees Celsius) can kill an unprepared human in just 30 minutes.

3. 1000 feet underground

8. The Cave of Crystals would naturally be filled with scorching water, were it not for industrial pumps that facilitate the mining of silver, zinc, lead, and other minerals in the caves.

9. More than a hundred giant crystals have formed in the two- story-tall, football-field-size Cave of Crystals.