When Shaun and Courtney Erk bought their house in Black Hawk four years ago, they went through the usual steps, like a home inspection.
“You know, there was nothing as far as a foundation issue.” Courtney said. “We had the radon test done. We had everything done you’re supposed to do.”
But none of that turned up the one thing the Erks really needed to know.
They finally learned that information one evening last month when Courtney came home from work. She found the neighborhood filled with firetrucks and cop cars.
There was a giant hole in the front yard of a house two doors down from them. They thought it was a sinkhole.
It turned out to be a collapsed mine.
Since then, cavers have mapped the mine. It sprawls under more than a dozen homes. Some of the caverns are up to 60 feet wide and 40 feet high. And in some places, they’re just 2 or 3 feet below the surface.
Public records show developers knew about past underground mining on the site when they built the neighborhood about 20 years ago. The mine is mapped on a U.S. Geological Survey database. Historical mining publications and newspaper clippings say the mine was producing gypsum as late as the 1930s.
Courtney Erk is outraged to learn all that now.
“Honestly, I’m almost speechless, really, that they allowed this,” she said.
It’s unclear how much the developers studied the risk posed by the mine, or how much consideration the county planning board gave it. The developer, Keith Kuchenbecker, did not return calls for this story. Neither did several other people involved in planning and approving the development.
The Erks are talking to a lawyer, and Courtney says her neighbors are, too. At least one group of affected residents is already suing, alleging negligence by numerous officials and developers, although the Erks are not part of that lawsuit.
There are about 40 people displaced by the mine collapse, according to Meade County Emergency Manager Doug Huntrods. Some are living in hotels or campers, and some are with friends and relatives.
Huntrods said none of the families’ insurance policies cover losses from a collapsed mine. They hope to recoup a substantial portion of each home’s value from a FEMA program. But that could take months or even a year or more. The Red Cross is coordinating with other groups to help the families in the meantime.
It’s unknown why the mine caved in, but there are theories. Karl Emanuel is a Forest Service geologist. He’s also part of the caving club, called the Paha Sapa Grotto, that explored and mapped the mine for authorities.
Emanuel thinks water may be the culprit. Underground water takes up space and can hold underground formations in place. He says any change in that – maybe a drawdown from pumping – can change things underground.
“When you draw that water out, things dry out, the roof starts to desiccate, and you get collapse,” Emanuel said during a recent presentation hosted by the caving club.
Emanuel said there are similar risks elsewhere in the Black Hills. That's because of the region’s mining history and geology.
The state’s abandoned mine database lists about 900 mines. But the state Department of Environment and Natural Resources said the database contains little information about the Black Hawk mine and doesn’t list an exact location. The DENR makes information from the database available by request.
Tim Cowman is the state geologist. He said mines in the database predate government regulations.
“We might know what section it was in or what quarter of what section it was in,” Cowman said, “but trying to peg it down to an exact location on those old historical mines is pretty challenging.”
Mine collapses aren’t the only subterranean risk to homeowners in the Black Hills. Many of the region’s geologic formations are prone to sinkholes.
Emanuel, the geologist and caving-club member, is using new aerial imagery to inventory sinkholes in the Black Hills. He started the work before the Black Hawk cave-in. The images are produced with lasers that are accurate down to several inches. The technique is called LiDAR, for Light Detection and Ranging.
Emanuel expected to find lots of sinkholes. There have been highly publicized sinkholes in the Black Hills, including one in a western Rapid City neighborhood last spring. But Emanuel is amazed at the high number of sinkholes he’s seeing. He’s already found more than 2,000, and based on that number, he expects to find 30,000 by the time he’s done.
“It’s just amazing what we have found since we have got this information,” he said.
Many of the sinkholes are in undeveloped areas. But 800 of them are in a clay valley between the Black Hills and the hogback ridges that surround the hills. The valley, known as the Spearfish Formation, includes Black Hawk and other fast-growing areas between Rapid City and Spearfish.
“It is riddled with sinkholes,” Emanuel said. “So if you live on the Spearfish Formation – and a lot of people do – it’s something to be considered.”
For those people, Emanuel recommends hiring a geotechnical firm to study the risk.
For the houses above the underground mine in Black Hawk, it’s too late. Authorities say many of the homes may need to be demolished. They think the mine is probably more expansive than anyone knows. Cavers couldn’t explore it all, because parts were flooded or collapsed.
Meade County is researching ways to learn just how big the mine is. That could include ground-penetrating radar, and the findings could mean more people will be evacuated. The state is trying to determine whether the mine might be a threat to nearby Interstate 90.
The county’s also studying ways to fill the mine, although that could be cost-prohibitive. For now, it’s blocked off by a fence as authorities try to figure out what to do with it.
-Seth Tupper is SDPB's business and economic development reporter.