Onyx Cave, National Natural Landmark solutional cave in Santa Rita Mountains, Arizona.
Onyx Cave is a limestone cavern in the Santa Rita Mountains containing passages that stretch roughly half a mile underground, formed over millions of years by water dissolving the rock. The interior displays mineral deposits and rock formations shaped by ongoing geological processes, with the onyx mineral giving the cave its distinctive appearance.
The cave formed roughly 300 million years ago when the region was beneath an ancient sea and limestone was deposited as sediment. Mid-twentieth century vandalism caused significant damage to the interior, prompting protective measures to be implemented.
The cave is named after the glossy onyx mineral visible in its walls, which caught the attention of early visitors. It has become a place where people come to experience underground geology firsthand and appreciate the fragility of natural formations.
Visit by advance reservation only, with strict guidelines to protect the cave's delicate formations and surfaces. Bring layers of clothing since the underground environment stays significantly cooler than outside, and wear sturdy footwear suitable for walking on uneven cave floors.
Protection came surprisingly late in 1974, after decades of damage from visitors had left lasting marks on walls and formations. This delayed recognition underscores how easily natural wonders can suffer when not guarded from the start.
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