The planet's crust crests like a wave at the twin peaks of Mt. Diablo, collapses north and crumbles to shore in Antioch, where the hills that form the city’s southern horizon jut steep and crowded as the shan shui of ancient China. Beneath their grasses, now golden in withering summer, those hills have kept a secret for countless millennia, a secret encoded in the ochre of sandstone rippled by the black of coal.

But the secret is out.

A century and a half ago, coal fed the furnace of the Industrial Revolution. So crucial was coal to the cause that it was dubbed “black diamond.” When in 1973 the East Bay Regional Park District (EBRPD) acquired the property in Antioch now known as Black Diamond Mines Regional Preserve, park officials knew what they were getting: the heart of Contra Costa’s historic coal and sand mine workings. In the words of EBRPD General Manager Bob Doyle, “We discovered a park with a lot of holes in it.”

Nearly 40 years later, those workings have been transformed into one of the most awe-inspiring excursions in the West.

Step inside

Nothing in our experience has prepared us for this. As we pass through the Hazel-Atlas sand mine portal and step from the bright heat of summer into the dim coolness of the mine, our primal fear of darkness and entombment collides with our primal urge to explore the mysteries of the earth. A strange aroma – an accent of sand and wood – permeates the faintly moist air of the tunnel.

But this place is far from dark and dank. Our eyes adjust immediately to the lamps lining the tunnel walls. Sturdy wooden buttresses reassure us that we’re in a safe place. We don our hard hats and strike out into the arched corridor, plunge into the planet and into the past. It’s a past marked by more than veins of coal and fossils of ancient sea creatures imbedded in the tunnel walls. It’s a past echoing with the voices of people.

To those who say “we know so much more than the people of the past” we say “yes – and they are a vital part of what we know.” Nowhere is this principle more powerfully felt than at Black Diamond Mines, where a tryst with history requires nothing more than reaching out and touching it.

Palpable past

In its golden era of the mid-to-late 1800s, the Mount Diablo Coalfield was the site of California’s largest coal mining operation, and the area we call Black Diamond Mines was the population center of Contra Costa County. Into the slope of Rose Hill, a half-mile northwest of the Hazel-Atlas portal, rest the bones of those whose fate was to dig out the coal – some 200 residents of the long-deserted towns of Nortonville, Somersville, Stewartville, West Hartley and Judsonville.

The price of mining nearly 4 million tons of the black diamond from hundreds of miles of underground workings was steep. Cave-ins, gas explosions and machinery-related accidents were common. In July of 1873 a miner named David Williams, working 700 feet underground, was standing 400 feet beneath a loaded coal car being hauled up the Mount Hope Slope when the car’s cable broke. As the Antioch Ledger put it, “… the loaded car with terrible momentum went to the bottom again, crushing the man Williams to atoms.”

Children weren’t exempt from the mine’s brutal agenda. “Nobbers,” boys 8 to 12 years old, worked behind the miner in a “coal room” angled upward into the coal seam. These rooms were up to 600 feet long and in some places no more than 18 inches high. The nobber would slide the loose coal down the coal room floor into an ore car in the gangway. Nobbers worked 12 hours a day, six days a week.

From coal to sand

The beginning of the 20th century saw the emergence of a new energy source – oil – as the chief rival of coal. The rush to petroleum power plus the mediocre quality of the Black Diamond seam’s lignite coal (inferior to the anthracite mined up in Washington), conspired to force the closing of the Diablo Coal Field.

But these hills weren’t finished disgorging their riches. High-grade silica sand, used in the making of glass, became an object of desire at Black Diamond from the 1920s through the ’40s. And that’s where our mine tour takes place: in a section of the sand mine workings of the Hazel-Atlas Company, which shipped the sand to its Oakland factory – one of many Hazel-Atlas facilities in the United States – for the manufacture of pressed glass.

If the Hazel-Atlas mine echoes with the voices of our predecessors in East County, like any mine in the world it also echoes with the voice of human civilization at its most nitty-gritty. “People forget how much everything they use every day comes from mines,” said Doyle. “The material in your cell phone comes from a mine.”

Whether for salt or silver – or the copper for your cell phone’s circuitry – humans have been boring into the earth and scooping out the raw materials of their material lives for more than 40,000 years. Take a look around the room you’re sitting in. Do you see glass? On the walls of the Hazel-Atlas sand mine, you get to see glass in its most basic form.

Walk of wonder

The raw materials beneath Black Diamond haven’t been lounging around in their present form forever. The relentless rhythms of Earth’s crust have lifted and twisted the landscape; the ocean has washed over it, retreated and returned. The flora of swamps and floors of seas, compressed over 50 million years, have become coal and sandstone – the stuff of civilization.

By the time human hands began digging into it, as Mine Manager John McKana put it, “The whole area was like a big layer cake tilted at 30 degrees and shuffled up 29 feet at the Hazel-Atlas Fault. The layers alternate from silty, dirty sandstone to shale to a coal seam to better sandstone and so on.”

The result is a realm of wonder, where you can run you fingers across exposed strata in snaking patterns frozen in time; peek through openings in the tunnel wall into the stopes, a chain of mini-cathedrals hollowed out of the hill’s sandstone core.

Grand plans

The length of the Hazel-Atlas mine tour is currently 950 feet. Though 950 feet of the park’s 150 miles of mine workings might not sound impressive, it’s an ambitious stretch. “All the ‘holes’ we use here were made by the sand miners,” said McKana. “But the park district had to timber the entire area to make it safe.”

More ambitious yet is the extended tour. As things stand now, the tour proceeds those 950 feet into the mine and returns along the same route. When the extension is open for business, said McKana, the tour will be a true portal-to-portal, looped excursion, beginning at the Hazel-Atlas Portal and concluding at the Greathouse Portal approximately 1,150 feet down – and around – the pike. Among the adjustments needed before the extended tour is ready to roll, according to McKana, is the rerouting of the stairs that descend to the Greathouse Portal.

Also down the pike – “in the conceptual stage,” said McKana – is a development that will put the “black diamond” back in the touring business: a replica coal mine. If you’re wondering why the park doesn’t simply open up a section of the old coal mine workings, the answer is simple: too dangerous.

Also planned is the construction of a 200-seat underground auditorium named after the late John Waters, who served the district from 1968 to 2006, was Black Diamond Mines’ first mine manager and a champion of mine safety and park development at Black Diamond.

Get on board

Black Diamond Mines is located at the southernmost end of Somersville Road in Antioch. When the entrance kiosk is attended (seasonally, on the weekends and a few holidays), a $5 parking fee is charged. Proceed past the kiosk and park offices to the parking lot where the road ends, snatch a map at the trailhead and proceed to the Greathouse Visitor Center. To secure a spot on the tour, sign up at the center about an hour ahead of time. Tours are conducted on the weekends from March through November at noon and 3 p.m. The fee is $5. Weekday tours for groups can be arranged by calling 888-327-2757, option 2. For their safety, children under the age of 7 aren’t allowed on the tour.

There’s plenty to do between sign-up and the beginning of the tour. At the visitor center you can view a video of the history of Black Diamond Mines, check out artifacts on display and browse through literature on local flora, fauna and a host of other subjects. The staff is knowledgeable, friendly and willing to answer your questions at length.

You still have time to do some sightseeing before the tour. Your mission, before the tour begins, is to check out points of interest four, six and seven on your trail map. The nearby Eureka Slope, Powder Magazine and Stope will give you a vivid flavor of the lay of the land. Bring a flashlight and you’ll be amazed at the dimensions of the Stope.

Adventurers underground should bring a jacket or sweater – even in summer. The weather forecast in the mines year-round: sunless but well lit; temperatures in the mid-50s. Natural wonders in natural air-conditioning.