Deep beneath the Chihuahuan Desert in Naica, Mexico, lies a geological marvel that defies imagination: the Cave of Crystals. Discovered in 2000 by miners searching for ore deposits, this extraordinary cavern is filled with colossal gypsum crystals, some of the largest natural crystals ever found. Imagine entering a hidden realm where beams of translucent selenite, a form of gypsum, stretch longer than telephone poles, creating an awe-inspiring spectacle of natural art.
The Cave of Crystals, located 290 meters (950 feet) underground, is nestled within a mountain rich in minerals like lead, zinc, and silver. Since its accidental unveiling by the Industrias Peñoles mining company, this subterranean chamber has become a magnet for scientists and explorers worldwide. They are drawn not only to its breathtaking beauty but also to unravel the scientific mysteries behind its formation and preservation.
Among those captivated by the cave is Juan Manuel García-Ruiz, a crystallographer from the University of Granada, Spain. For García-Ruiz, who has been growing crystals in labs since his youth, witnessing the Naica crystals firsthand was a dream realized. He described his initial experience as euphoric, a moment of pure awe and wonder.
For nearly two decades, researchers like García-Ruiz have ventured into the cave’s intensely hot and humid environment to probe the origins and growth mechanisms of these giant crystals. Now, with many fundamental questions answered, the focus is shifting towards strategies for protecting and conserving this unique geological site for generations to come, contingent on the ongoing activities within the mine and the mountain itself.
The Birthplace: An Underground Flask
Around 26 million years ago, geological forces deep within the Earth pushed magma upwards beneath what is now southeastern Chihuahua, Mexico. This upwelling magma created the mountain near Naica and infused hot, mineral-laden waters into the existing caverns and fissures within the limestone bedrock. It was within these mineral-rich waters that the extraordinary Cave of Crystals began its formation.
The cave became saturated with water rich in calcium sulfate. While calcium sulfate can crystallize into various minerals, it was gypsum (CaSO4·2H2O), specifically the transparent, colorless variety known as selenite, that became the dominant mineral in these unique cave conditions.
Exploring the breathtaking scale of the giant gypsum crystals within the Cave of Crystals in Naica, Mexico. These massive formations highlight the unique geological conditions that allowed for their extraordinary growth over millennia.
“Every mineral exists within a stability zone,” explains Alexander Van Driessche, a crystallographer at the National Center for Scientific Research’s Institute of Earth Science. Van Driessche, who collaborated with García-Ruiz during his postdoctoral research, studied the Naica crystals to understand their formation process. Anhydrite (CaSO4) is more stable at temperatures above approximately 58 °C, while gypsum is more stable – meaning less soluble – below this temperature.
Initially, deposits of anhydrite formed in the hot, magma-heated waters. Over thousands of years, the water gradually cooled. As the temperature dipped below 58 °C, the anhydrite began to dissolve, and gypsum crystals started to nucleate and grow. The dissolving anhydrite provided a consistent supply of calcium and sulfate to the subterranean solution, maintaining a state of slight supersaturation, which are ideal conditions for the slow and steady growth of gypsum crystals (Geology 2007, DOI: 10.1130/G23393A.1).
Crystal growth, whether in a laboratory or a natural cave, always begins with nucleation. This is the process where the crystal’s basic molecular components arrange themselves around a minute initial structure and begin to expand. For instance, tiny particles in the atmosphere act as nucleators for ice crystals, which eventually become snowflakes. In ionic solutions, high supersaturation encourages the formation of many crystals, while low supersaturation promotes fewer crystals but allows for larger growth. The Cave of Crystals presented the perfect balance: conditions that favored minimal nucleation but maximal growth, enabling the few initial crystals to reach gigantic sizes.
Intrigued by the colossal Naica crystals, Van Driessche and his colleagues conducted laboratory experiments to study gypsum nucleation. They made a surprising discovery: gypsum crystals form through an unusual nucleation process. Instead of starting from a tiny gypsum crystal seed, they grow from nanoclusters of CaSO4 that coalesce (Science 2012, DOI: 10.1126/science.1215648). This finding challenged previous understandings of crystal formation.
Understanding the fundamental mechanisms of gypsum crystal formation has implications far beyond the caves of Mexico, according to Van Driessche and García-Ruiz. It could offer insights for preventing mineral buildup in desalination plants or for understanding gypsum formation on Mars. However, their curiosity extended beyond nucleation to the question of how the Naica crystals attained their immense size.
The Secret of Immense Growth: Infinitesimal Expansion
The Cave of Crystals isn’t the only cavern beneath the Naica mountain containing gypsum crystals. The same mineral-rich waters permeated other nearby underground spaces. Yet, the crystals in these other chambers did not reach the same extraordinary dimensions. Van Driessche and García-Ruiz concluded that, in addition to the Goldilocks conditions that initiated crystal formation, a precisely slow cooling rate was crucial for the crystals to achieve their mammoth size.
The intricate crystal structure of gypsum, composed of calcium (blue), sulfur (yellow), oxygen (red), and hydrogen (pink) atoms. This layered structure, with water molecules sandwiched between calcium sulfate layers, is key to understanding gypsum crystal growth and properties. Credit: Cryst. Growth Des.
The Cave of Swords, situated at a shallower depth of 120 meters (390 feet) within the same mine, provides a stark contrast. As its name suggests, its walls are densely covered in shorter gypsum crystals, resembling medieval swords, reaching up to 2 meters (6.5 feet) in length. In the deeper Cave of Crystals, the water cooled at a considerably slower pace than in the Cave of Swords. This gradual cooling over vast periods meant that only a few gypsum crystals nucleated, but these had the opportunity to grow to enormous proportions, explains Van Driessche. Conversely, the faster temperature drop in the Cave of Swords resulted in a greater number of smaller crystals. This difference perfectly illustrates the principles of crystal nucleation and growth, he notes.
Because the water temperature in the Cave of Crystals remained within the delicate transition zone between anhydrite and gypsum for an exceptionally long time, the crystals grew continuously, undisturbed and undiscovered, for eons. But exactly how long? The researchers were determined to find out, but determining their age directly proved challenging. The high purity of the crystals meant that isotope-dating techniques, which rely on trace elements like uranium, could only provide a rough age estimate. Instead, Van Driessche, García-Ruiz, and their team meticulously measured crystal growth rates in the lab using samples of the crystals and water from the Naica mine.
The layered structure of gypsum crystals—calcium sulfate layers interleaved with double layers of water molecules—simplified this task. The hydrogen bonds between the water layers are easily broken, allowing thin flakes to be readily removed from the crystals. This provided pristine, flat surfaces ideal for growth studies, Van Driessche explains.
“When I entered the first time, after the first couple of minutes of stupor, I burst out laughing. I was euphoric.”
Juan Manuel García-Ruiz, crystallographer, University of Granada
By immersing these pristine gypsum samples in water from the mine for 24–48 hours, the team measured the increase in height of the flat surface using phase-shifting interferometry. This light-based technique is sensitive enough to detect surface growth rates as slow as 10–5 nm/s. By conducting measurements at various temperatures, the scientists estimated the growth rates of the Naica crystals within their formation temperature range, approximately 54 to 58 °C. With this data, they calculated the time it would take for a 1-meter-thick crystal beam to grow at 55 °C.
The result was astounding, even considering the expectation of extremely slow growth. Such crystals would have taken nearly 1 million years to reach that size (Proc. Natl. Acad. Sci. U.S.A. 2011, DOI: 10.1073/pnas.1105233108). This incredibly slow growth rate is equivalent to adding the thickness of a sheet of paper every 200 years, Van Driessche points out.
Drained and Delicate: The Cave’s Future
Had the crystals remained submerged in the mineral-rich water, there would have been no limit to their potential size. Current conditions at that depth, with a water temperature around 55 °C, are still conducive to gypsum crystal formation and growth, according to Van Driessche. Small ponds within the mine even contain newly formed gypsum crystals. However, years of mining operations in Naica have artificially lowered the water table to access more lead, zinc, and silver deposits. While draining the cave revealed the giant crystals, it also exposed them to a harsh environment and introduced risks to their stability.
During mining operations, Industrias Peñoles pumped water out of the mountain at a rate comparable to filling an Olympic-sized swimming pool every 40 minutes, creating a small artificial lake near Naica. As the water level decreased, more caves and tunnels were exposed to air—and human access. The Cave of Crystals, due to its depth and recent discovery, escaped the extensive removal of crystals that occurred in the shallower Cave of Swords, discovered in 1910. Collectors had removed many of the largest and most beautiful crystals from the Cave of Swords.
Peñoles implemented strict access controls to the Cave of Crystals to protect both the crystals and visitors. The cave’s environment pushes human physiology to its limits. The temperature hovers around 50 °C (122 °F) with over 90% relative humidity. In these conditions, sweating provides no cooling effect, making even short visits challenging.
“I just want to see them once more.”
María Elena Montero-Cabrera, researcher, Center for Research in Advanced Materials
Navigating the cave is also perilous, explains María Elena Montero-Cabrera, a researcher at the Center for Research in Advanced Materials in Chihuahua. Researchers had to clamber over condensation-slick gypsum spears to explore the cavern. Precautions were essential to avoid getting lost or falling, as rescue operations would be incredibly complex and dangerous.
Image shows a person in the narrow Cave of Swords with swordlike crystals covering the walls on both sides. Credit: Javier Trueba/MSF/Science Source
Due to the extreme conditions, researchers could only endure 10–15 minute sessions inside the cave, Montero-Cabrera recalls. The cave is isolated from the rest of the mine by two sets of doors, safeguarding the crystals from external conditions and maintaining a tolerable temperature and humidity in the antechamber. Before each entry, a medical check was mandatory to ensure visitors were fit enough to withstand the cave’s climate.
Beyond the challenges for researchers, the drained environment poses a threat to the crystals themselves. The largest beams, estimated to weigh 40–50 metric tons, risk cracking under their own weight without the buoyant support of water. Gypsum is a soft mineral, ranking only 2 on the Mohs hardness scale (talc is 1, diamond is 10), and foot traffic has already worn a dark path into the crystals on the cave floor, Van Driessche observes.
To devise the best preservation strategy for future generations, Montero-Cabrera and her research team in Chihuahua studied the impact of the drained environment on the crystal surfaces. In a year-long lab experiment, they exposed samples of the Naica crystals to various gaseous and liquid environments to identify potential changes or impurities that could compromise the gypsum’s long-term integrity and appearance.
Their findings indicated that the crystals fared slightly better in a liquid environment, while gaseous environments led to dehydration. Specifically, they detected bassanite, a dehydrated form of calcium sulfate, on the surface of some gypsum crystals (Cryst. Growth Des. 2018, DOI: 10.1021/acs.cgd.8b00583). This suggests that the crystals’ appearance will gradually change over time. However, the team concluded that removing crystals from the sealed cave is not a viable preservation method.
An Uncertain Future, Enduring Wonder
Research activities in the Cave of Crystals have gradually decreased. Many fundamental questions have been answered, and the mining company closed access to the cave around 2015 due to a leak that flooded the mine faster than pumps could remove water. While the water level in the mine has risen since, it’s unclear if it has reached the Cave of Crystals.
Photo shows a person inside a cave containing large, clear gypsum crystals. Credit: Javier Trueba/MSF/Science Source
Montero-Cabrera notes recent reports suggesting that mining operations may resume through a different entrance, potentially allowing researchers to access the cave again. Whether the cave itself will flood again remains uncertain. In the meantime, researchers are turning their attention to other intriguing gypsum deposits worldwide, such as the crystals found in an 8-meter-long geode in Pulpí, Spain.
García-Ruiz and Van Driessche are currently studying the Pulpí crystals, which are smaller but more transparent than those in the Cave of Crystals. They aim to understand the reasons for these morphological differences and, in García-Ruiz’s case, to refine the age estimate of the Naica crystals.
Although Montero-Cabrera has shifted to other research areas, she would eagerly return if the cave reopens. “I just want to see them once more,” she expresses.
For now, the giant crystals remain isolated—a hidden, otherworldly wonder awaiting an unknown future, a testament to the slow, powerful forces of nature and a call for continued exploration and preservation of our planet’s geological treasures.
