Crystal Caves of Mexico
While excavating a tunnel under Mexico's Naica Mountain, silver miners were stunned to
discover a cavern full of luminous, elongated underground crystals. Composed of the
mineral gypsum, some of the crystals extended as far as thirty-six feet into the air. What
accounted for these amazing mineral structures? Millions of years ago, volcanic eruptions
released a large amount of mineral-rich water into the cave, now known as the Cave of
Crystals. One of those minerals was anhydrite, which slowly changes into gypsum when
exposed to water. Originally, the cave was too hot for this change to take place.
Eventually, however, the temperature cooled to about 136°F, and gypsum crystals began
to form. This process continued as the temperature and other conditions in the cave
remained constant for thousands of years.
What is the main idea of the passage?
Gigantic crystals in Mexico's Cave of Crystals were discovered by silver miners
who were excavating a tunnel.
Specific conditions in the Cave of Crystals led to the development of the
enormous crystal structures found there.
