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The unassuming entrance to the 2 million-year-old Black Chasm Cavern near the town of Volcano barely hints at the secret treasures that await below. In order to access them, visitors must navigate 162 stairs in an otherworldly 100-foot descent.

Just a few steps down the first stairwell, a temperature transformation is immediately felt. Designated a National Natural Landmark because of its abundance of rare helictite crystals, the privately run cavern’s consistent sub-60-degree temperature — no matter how hot or cold it is outside — can be jarring, especially on this 90-degree afternoon in early June.

“It’s always at 57, 58 degrees,” says guide Kristopher Larson, who puts on a jacket before beginning the 50-minute walking tour. “In the wintertime, (the constant temperature) is more the cooler air is pushing down, because the air is always trying to equalize. But in the summertime, that cold air doesn’t rise, so the hot air is kind of, like, hitting a boundary.” Likely discovered by local Miwok people before the Gold Rush era and then first explored beginning around 1854 by miners, Black Chasm is a naturally formed vertical (or solution) cave, with its main structure comprised of limestone and marble, also trace elements of iron, manganese and chromium. “It’s created when the Sierra mountains rise up from sea level and hollows out inside the cave with either natural erosion or carbonic acid, and it creates calcite formations inside the cave,” says Larson, who’s guided at Black Chasm for three years. “That’s the reason you come into the cave and see the beautiful formations. The average tourist gets to see 10 different formations, from stalactites to helictites.”

Stalactites, which descend vertically, and helictites, which extend horizontally or diagonally, are rare icicle-shaped formations that hang from the ceiling of a cave or branch out and grow in several different directions. They’re the result of the precipitation of minerals dripping through the cave ceiling, and helictites are found in only an estimated 5 percent of known caves on the planet — just three in the U.S., according to our guide.

Inside the cave from one of the three platforms and five flights of stairs, tourists can take in the view of hundreds of thousands of formations, including some that twist into shapes that look like different animals and figures such as candy canes, butterflies, reindeer and a dragon. The dragon is considered the cave mascot and found in statues in the cavern’s gift shop.

After the Gold Rush, the cave was mostly abandoned, though basic tours were led periodically by a few entrepreneurs in its smaller, front chamber. However, the cavern was still being studied by UC Berkeley speleologists, and the 1976 federal designation was followed about 20 years later when it was fully developed as a show cave with the construction of an extensive array of steps and walkways.

Black Chasm is currently operated by Gold Country Adventures, Inc., which also runs the California Cavern tours in Mountain Ranch about 30 miles away. (California Cavern has more of a horizontal layout.) Guided tours at Black Chasm — with a maximum of 20 people per tour — run daily and are scheduled hourly throughout the year.

While the tour goes 100 feet below ground, the total depth of the cave is 265 feet, Larson says, pointing out that at the 200-foot mark, there’s a 64-foot-deep lake system barely visible to the naked eye. It contains just one species, the shrimp-like amphipod, which has lost its coloration and its eyesight. There’s also a tiny spider that’s only native to Black Chasm, called Banksula grubbsi. It’s similar to a daddy longlegs spider, Larson says, but measures only 1.5 to 2 millimeters.

In addition to the cavern tour, visitors can take a couple of free hikes located near the visitor’s center, including the mile-long “out-of-this-world-looking” Zen Garden Nature Trail. Over 75 feet of dirt was removed there during mining days, exposing stunning bedrock formations, says Judy Wilson, who supervises the approximately 20 employees at Black Chasm.

While California Cavern closes down in the winter because of flooding, Larson says Black Chasm always stays open, even during heavy rainstorms, because the water doesn’t rise to a dangerous level for visitors. As a result, the versatile Black Chasm site is a popular stop for school tours, sometimes running up to 220 students a day through the cavern.

“We do all types of events here in the cave. We’ve even done weddings,” Larson says.

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