Thick darkness, indeed. I’m glad that the headlamp on my helmet has replaced Muir’s candles, but I’m still amazed by the nearly palpable darkness. This is an absence of light so complete there is no above-ground equivalent. Turn off the headlamp, and there is no difference between having my eyes open and having them shut: I can’t see my companions or even my own fingers. Stay this way long enough, warns our guide, and we’ll begin to doubt our own existence. The weird appeal of this Samadi-tank state is partly what attracts experienced cavers to the self-imposed deprivation of a subterranean environment. A cave erases your above-ground sensibilities and replaces them with a roil of emotions—exhilaration and fear and triumph tumbling together like granite boulders.

Even within the safe format of a guided tour, navigating the contorted tunnels of California Caverns is a physical and psychological challenge, as I discovered during a field trip with the museum’s Natural Sciences staff. To research the exhibit California Under-ground: Our Caves and Subterranean Habitats, the group wiggled through claustrophobic "squeezes"—mere slots in solid rock, slid through channels lined with mud and marveled at calcite deposits that glowed, opalescent, from dim cave ceilings. In the end, we learned as much about ourselves as about the cool, mysterious chambers winding through California’s geologic crust.

A cave is not so easily revealed. It unfolds. It lures. It touches one’s own dark places. No wonder the subterranean always has been a realm of mystery and a metaphor for the unknown. Every culture has its underworld mythology, rarely pleasant, usually as damp and dark as fear, and for good reason. As I’m about to learn, a cave demands actions no sane human would commit in the light of day.

The summer’s heat has painted the valley gold. Our group heads to the Sierras, past the Gold Rush towns of Copperopolis, Murphys and Angel’s Camp, to California Caverns, the first developed "show cave" in the state and one of the few public caves that hasn’t been ruined by visiting crowds. By the time we arrive, an hour before noon, the day is burning hot. We zip coveralls over our jeans and try on our hard hats, looking forward to the natural air conditioning awaiting inside the cave.

Wafts of cool air emanating from heated rock are an age-old clue to the presence of an underground cavern, explains our guide, Art Holly, as we trudge, already sweating, to an entrance hidden behind hillside boulders. The native Miwok knew this. Depressions in the rock near some of the cracks and crevices in the earth illustrate that centuries ago women sat here to grind acorns and enjoy the frigid exhalations. The Miwok probably ventured no further, however, believing that the nether world was a place of departed or powerful spirits, a place to be respected, not invaded.

A gold-seeker known as Captain Taylor, who paused on this hillside in 1850, had no use for spiritual deterrents. He, too, felt the tell-tale breezes, and promptly decided that a pathway to riches lay within. He staked a claim, chipped away the boulders, and began to explore the labyrinth of tunnels. In the end he didn’t find gold, but he did discover a rich resource for amateur cavers.

There are two entrances to the caverns. One is protected by a metal door and admits walking tours of the "Trail of Lights," a network of developed paths accessible to the public. Electric wires snake audaciously over the rock, powering spotlights every few yards, bathing the cave in a stagy, artificial glow that’s somewhere between Mother Nature and Walt Disney.
We wiggle down the more secluded caver’s entrance, a slippery corkscrew tunnel that opens into a huge granite chamber. Here, we pause as Art explains that this rock once was a coral reef in an ancient ocean. Forces of continental drift lifted this shelf of rock at some prehistoric point in time, folding it into Sierra Nevada granite like a geologic cake batter. Groundwater enriched with carbon dioxide etched tunnels, filling them with a solution of water and dissolved limestone. When earth movement eventually opened the tunnels, allowing air to penetrate, the carbon dioxide dissolved and the limestone solution dripped out of the rock, forming the calcite deposits for which the cave is famous.

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