Just 100km south of Mexico’s iconic Parral mining district in Chihuahua lies the world-class Naica mine. The project, also known as Maple, operated for 64 years. It was first acquired by Fresnillo in 1951 and in 1964 changed hands to become an official part of sister company Industrias Peñoles’ portfolio. With its own concentration plant, the mine was the country’s second-largest lead mine. In its final year of production – 2014 – its output equated to 21kg of gold, 57,585kg of silver, 19,694 tons of lead and 15,399 tons of zinc. In 2015, with declining production and a drop in base-metal demand, Industrias Peñoles decided to suspend operations at the Naica mine indefinitely.
But beyond the precious metals and base minerals held within Naica, there are less typical reserves. In the year 2000, the Cueva de los Cristales, or Cave of Crystals was discovered by two Peñoles miners 290m below surface level, within the mine’s limestone host rock. The cave is home to selenite crystals that are five times larger than any ever discovered, measuring up to 13m and weighing more than 55 tons. Discovered more recently and closer to the surface, at a depth of approximately 120m is the Cueva de las Espadas, or Cave of Swords. The crystals held within this chamber are much smaller, at around 1m long due to the fact that they are much younger than those contained in the Cave of Swords.
The recorded temperature in the Cave of Crystals is 45°C, with humidity levels of 80 percent, which makes it impossible to breathe and increases the risk of losing consciousness in less than 10 minutes. Researchers must wear oxygenated suits in order to be able to withstand 30 minutes of continued investigation. In fact, the cave is closed to the public for this reason. According to National Geographic, “the crystals are searing hot to touch; razor sharp but also soft like human fingernails.”
The crystals require two conditions to grow: water immersion and heat in excess of 48°C, so scientists believe that conditions underwater must have been stable for hundreds of thousands of years. With the cessation of mining activities within the mountain, the water would be returned to the cave to once again stimulate growth. In March 2017, NASA scientists discovered microbes that had been dormant in the mine for between 10,000-50,000 years, which they believe demonstrates the ability of lifeforms to adapt to more hostile environments. This, they say, is a step forward in humans finding another habitable home within the solar system.