Mine tour explores the belly of Mount Diablo

The view down a chasm from a wooden bridge 900 feet from the entrance of the Hazel-Atlas Mine. Visitors in hard hats took a 90-minute trek 950 feet into the belly of Diablo's foothills to observe the geology and learn about mining.

ANTIOCH, Calif. - They call it a portal, but, really, it's more like a gaping maw. The opening's padlocked, steel-reinforced gates take on an almost predatorlike aspect, a gangster's grillwork sneering back at you. A dozen of us stood before this threshold, awaiting entrance, staring at the concrete portico bearing the chiseled inscription 1930, and trying not to obsess over minute cracks in the foundation.

Our guide for the morning, Mickey Rovere, sought to reassure us - and by us, I mean just me and maybe that wary 8-year-old in the back - of the safety of the Hazel-Atlas Mine, a long-dormant operation that extracted first coal, then silica, from the foothills of Mount Diablo. But in the very act of assurance, in his repeated furrowed-brow recitation of preventive measures and liberal use of the phrase structural integrity, Rovere only heightened my anxiety.

"We have modern-day miners in here maintaining these mines every day," Rovere said. "They are testing the integrity of the rock to ensure that where we're going to be walking is going to be safe. But it is a state requirement that we keep hard hats on. Remember, this is a real mine. The hard hats should be a reminder that you are definitely underground, OK?"

Gulp. I have a thing - irrational, I know - about enclosed spaces. I fear depths, not heights, harbor debilitating entombment fantasies, imagine all sorts of worst-case, buried-alive scenarios, akin to what those 33 Chilean miners faced a few years back. Sure, the opening of the shaft looked secure enough, inviolable even, as we peered in while Rovere fumbled with the padlock and opened the double-steel gates with an eerie creak. Structural integrity, I told myself. Structural integrity. It became a mantra, a lullaby to combat the decanted cortisol, that toxic stress hormone, flooding my neural pathways.

No one else appeared the slightest bit fazed, not even the 8-year-old. Their fascination with getting a glimpse of history about a bygone energy-extracting industry, as well as gawking at glittering sandstone rock 50 millions years in the making and fossil remnants from what once was the ocean floor, seemed to override any deep-seated angst. After all, people have been touring the mine at Black Diamond Mines Regional Preserve for two decades, not counting that closure from 2007 to 2012 after storm damage threatened the (wait for it) structural integrity and necessitated extensive repairs under rigorous mine-safety regulations.

Once inside, wearing hard hats and clutching flashlights like life preservers in the pleasant 57-degree coolness, we prepared for a 90-minute trek 950 feet into the bowels of Diablo's foothills to observe the geology and learn how, from the late 19th century to the late 1940s, miners blasted into the rock to scrape away first coal and then, when that rich vein ran dry, 1.8 million tons of silica from the sandstone to ship to Oakland for making glass.

But first, Rovere gave us the lay of the land via a slide show in a room chiseled out from the main shaft. In a feverish 10-minute presentation, the peppy, fast-talking Rovere took us from the early Cenozoic Era, when Mount Diablo was ocean floor, through tectonic upheaval and formation of coal deposits. Then he segued into the entrepreneurial forays in which four million tons of coal were pounded out by workers, some just kids, for about $3 a day, while risking black-lung disease, boiler explosions, and spontaneous combustion. Then came the sandblasting stope days of silica mining, in which seven layers of parallel tunnels snaked more than seven miles, connected by railroad tracks.

By the time he got to the closing of the mine, Rovere, almost as an aside, told us "the reason we became a park" - which did nothing to quell my anxiety.

"These mines produce gases - methane, carbon monoxide, carbon dioxide - and, after the mines closed, a lot of young explorers wandered into them and couldn't find their way out. These gases were overwhelming. So . . . [the East Bay Regional Parks District] came in for public safety, sealed off all of these coal mines using special foam and concrete, and opened this one, making sure the structural integrity was sound.

"What I like to call this is an 89-year-old sand castle . . . except we don't have a tide that comes in and washes us away. We have state-certified miners who come in and gently tap on these walls and ceilings - we call it scaling - all week long to test the structural integrity. OK, any questions?"

There's always one in every group, some dolt who holds everyone back from actual exploring with an alarmist question. That person happened to be me. I asked about earthquakes.

"We're actually going to walk right through a fault," Rovere said. "In 1989 [the Loma Prieta quake], we had just one rock fall."

"Oh, that's nothing," I said, relieved.

"It was a 700-pound rock, so it wasn't nothing," he said. "It came out like a loose tooth right over there."

He pointed to a spot near the portal, saying, "It crushed all our [equipment]. That's why this area is all reinforced concrete, to protect walls."

As we trudged ahead about 30 feet through the tunnel, about 8 feet high and 10 feet wide, the concrete and wood reinforcements ended. We were surrounded by sandstone. I brushed my shoulder against a pillar, and sand crumbled down my arm. Such porousness didn't inspire confidence in, well, the structural integrity of the walls.

Rovere stopped us at a re-creation of a Brass Board, where miners would punch in and the boss could see where in the bowels of the pit they were toiling.

"That boss is responsible for my life as a miner," Rovere said. "As I mentioned, lots of accidents happen. We're using dynamite. We're blowing up rocks.

"That's our job. Rocks fall - on purpose. They hurt, too. OK, let's move on."

Over the next 30 minutes, Rovere showed us all aspects of sand-mining operations: the cavernous stopes where miners toiled, chutes where rock traveled on its downward tumble to the rail carts, ominous-looking machines called muckers made to clear the rubble with a catapultlike lever, pipes used to carry water to keep the dust down after blasts, holes left by six-foot drill bits from pneumatic augers, holes left by "prehistoric burrowing ghost shrimp" in walls that formerly were the ocean floor, the office where the pit boss kept an eye on the number of carts chugging by.

"Our job here was simple," said Rovere, who had sort of internalized the experience and deputized us as fellow miners, a tad unsettling for a guy who has had only desk jobs. "We had to haul out as much . . . earth as we possibly could without letting the hillside collapse. We had to leave behind these large structural pillars to keep the integrity of the hillside."

We shuffled on, following the rail tracks deeper in, the space lighted by bare bulbs. Rovere had us shine our flashlights on a large bulge of white silica sandstone, "95 percent pure silica oxide," and the light reflected back at us, delighting the 8-year-old in the back.

I thought it was pretty cool, too, and I felt my anxiety quell a bit.

"When that silica oxide is melted down with soda ash and limestone," Rovere explained, "you've got the clearest glass available in that era - Depression glass, 1924 to '46. Made a lot of Mason jars for California produce."

Hazel-Atlas Mine Tour, Black Diamond Mines Regional Preserve, 5175 Somersville Rd., Antioch, Calif. Hours: Visitors center open 10 a.m. to 4:30 p.m. weekends from March to November. Tours, $5; parking, $5. Information: 510-544-2750, ebparks.org/ parks/black-diamond.