An accidental underworld
In April 2000, miners working for the Industrias Peñoles company were drilling a new access tunnel 300 meters (980 feet) beneath Naica, Chihuahua. They were searching for lead, zinc, and silver deposits when they broke into a horseshoe-shaped cavity in the limestone. Inside was a chamber filled with massive, milky-white crystal beams. The miners, brothers Juan and Pedro Sánchez, were the first humans to ever see the Cueva de los Cristales, or Cave of Crystals.
The environment they entered was extreme. Air temperatures reached up to 58°C (136°F) with 90 to 99 percent humidity. Under these conditions, a person without protective gear can only survive for about ten minutes before facing fatal heatstroke. The humidity is so high that fluid would condense inside the lungs, making breathing difficult. For the next decade and a half, scientists could only explore this environment for short periods, relying on specialized cooling suits packed with ice. The only reason the cave was accessible at all was because of the mine's powerful pumps, which constantly drew water out to keep the tunnels from flooding.
A half-million year recipe
The giant pillars are crystals of selenite, a transparent variety of gypsum (CaSO₄·2H₂O). Some of the largest natural crystals ever found, the biggest examples reach 11 to 12 meters (36 to 39 feet) in length, 4 meters in diameter, and weigh an estimated 55 tons. Their existence is the result of a incredibly stable set of geological circumstances.
For at least 500,000 years, the cave was filled with mineral-rich groundwater, heated by a magma chamber deep below the mountain. The water remained at a near-constant temperature just below 58°C. This specific temperature is the transition point where a mineral called anhydrite, which was abundant in the heated water, dissolves and recrystallizes as gypsum. Because the temperature remained so stable for such a long period, the gypsum formed slowly and with immense structural perfection, allowing the crystals to reach their gigantic sizes. Had the temperature fluctuated, it would have created smaller, less perfect crystals.
In October 2015, after mining became unprofitable, the pumping operations were suspended. Water from the aquifer began to fill the mine and the Cave of Crystals once more. This re-flooding, while making the cave inaccessible, effectively seals and preserves the giant crystals. Exposed to air, the soft selenite can degrade; submerged in the mineral-saturated water that created them, they are protected and may even resume their half-million-year growth.