Down the sinkhole

Three Custer cavers who have logged thousands of hours exploring and mapping caves around the country got to put their experience to a unique use recently. The trio were part of a group of Black Hills cavers who crawled down a massive sinkhole in a Black Hawk neighborhood to map the underground void that was endangering as many as a dozen homes in the subdivision. 

Homes in the Hideaway Hills subdivision between Sturgis Road and I-90 in Meade County had to be evacuated Monday, April 27, after sinkholes swallowed up chunks of lawn and a sidewalk. Public officials looking to find the source of the holes sought how they could determine the extent of the underground void and determine just how many homes might be threatened. 

Enter Paha Sapa Grotto, a group of local cavers (or “spelunkers,” as they used to be called) who offered to crawl into the newly-opened cavern to see what was down there.

After three members of the group made an initial survey, a party of eight Grotto members, including Custer residents Chris Pelczarski, Rene Ohms and Dan Austin, went underground the evening of April 30 to try to determine the extent of the void. 

What they found was an abandoned gypsum mine with extensive passages and ceilings as high as 15 feet. In addition they found a 1951 Ford car, ore cars and tracks and pools of water. 

Pelczarski, who has been exploring and mapping natural caverns like Wind and Jewel caves for nearly eight years, said going into the mine was quite different from cave exploration, partly because there was so much more room in the mine. 

Also, because mines are much less stable than natural caves which have been around for thousands of years, Ohms says the group exercised more caution going into the sinkhole than they normally would have for a caving expedition. 

In addition to their regular gear, which includes helmets, an array of bright lights and their mapping and survey tools, the group took along three gas meters to make sure the air was safe to breathe.

“It actually went off once when I started,” laughed Pelczarski, noting it was a false alarm. “For a second I was like, ‘Uh oh!’”

Once inside, the cavers discovered an underground world not seen by human eyes in many years. Using specialized equipment, the group measured the space, determining the mine was approximately 650 feet long and between 60 and 150 feet wide.

Pelczarski said the height of the ceiling was never over 15 feet and sometimes as little as eight feet.  

“It’s like a wide room with a bunch of pillars,” said Pelczarski. “The farthest you could see in one direction would be 150 feet.”

In at least two spots there were estimated to be only about two feet of rock and soil between the ceiling and the surface. 

“Those will likely be the next sinkhole,” said Pelczarski. 

Ohms noted that because of the porous nature of the gypsum (which is used to make sheet rock, among other things) the rock washes away over time, causing the ground to collapse into the opening underneath. A contributing factor to the recent collapse may have been the last two years’ abundant rainfall. 

“We were directly underneath several houses,” said Ohms, who noted the cavers believe they saw perhaps less than half of the total area of the mine. They were unable to get to other areas because of previous cave-ins.

“We know the mine is more extensive than what we mapped because parts of it are flooded and cannot be accessed and other parts of it are collapsed,” said Pelczarski. He believes the collapsing was either done intentionally when the mine was abandoned or has happened naturally over time.  

“There still remains a whole section of the mine you can’t get to because there are no sinkholes leading into it yet,” said Pelczarski, “but it seems like there will be at some point.”

He believes more houses are threatened because homes in the neighborhood located 10 doors from where the group stopped mapping are already having problems with their basements heaving.  

Besides the Ford, which the group believes found its way underground via a sinkhole that had developed previously, they also discovered a strange cone of fine gravel which nearly filled one passage. 

Ohms described the pile as “a giant mound of fill material 10 feet tall and 18 feet across and goes all the way up to the ceiling in a cone shape.” 

The cavers knew they were standing directly under a road and surmised that at some point there was a hole in the road which workers tried to fill by dumping in loads of fine gravel. 

“They had to dump in enough so the top of the cone would reach the hole on the surface,” said Pelczarski, who noted the base of the pile was as wide as the passage itself. 

The photographer for the group was Austin, who said photographing the mine was a challenge primarily because of the whiteness of the gypsum which reflected the light of his multiple flash units. 

While he ended up with only about 10 satisfactory underground photos, they have been widely featured on TV news, newspapers and websites over the last week. 

Austin joined the caving club in 1996 at the age of 16 and has since explored and mapped caves all over the country including one on private property near Nashville, Tenn., which he helped map for the owner. 

While he got his start in underground exploration by going into old area mines with his father, he now tries to steer clear of manmade holes because of their instability. 

He says he definitely used more caution in climbing into the Black Hawk sinkhole, examining the opening for loose soil or rock and avoiding obvious cracks in the ceiling. 

“Any time there has been a recent collapse, it calls for more caution,” said Austin, “especially near the entrance.” 

“We feel really bad for everyone in that neighborhood,” said Ohms, adding that it “feels really good” to be able to at least help them determine the extent of the risk. “We’re used to making maps of underground spaces for the scientific understanding of what’s down there in these big cave systems. This just brought it to a whole other level to be mapping these spaces under people’s homes and contributing to  information that’s going to help them figure out who to evacuate and who’s at risk.”

Officials in Meade County are investigating the 2002 development of the property to determine how the presence of the mine was overlooked when the subdivision was approved. Authorities are also looking into securing a Federal Emergency Management Agency (FEMA) grant to compensate homeowners 75 percent of the value of their homes before the emergence of the sinkholes. 

Pelczarski said, while the cavers knew they were directly under some people’s homes, he never heard anyone walking around “upstairs” but said they could hear the traffic on nearby I-90 which he called “surreal.” 

“It was a unique experience,” said Austin. “I didn’t ever expect that I would be surveying in a mine.”