As time marches on, portions of the Quad-Cities' underworld are expected to reveal themselves.
It happened 20 years ago in Moline.
An eroding hillside reopened a shaft to a long-dormant limestone mine at the corner of 5th Avenue and 34th Street.
As you will read, the handful of people who rappelled down a tree root and into the massive enclosure haven't forgotten it.
Another cave, this one at the Mound Street and River Drive entrance to the Village of East Davenport, was used to store hundreds of barrels of beer. People today cannot agree on how it got there.
Throughout the Quad-Cities, coal and limestone mining were necessary and profitable operations in the late 1800s and early 1900s. One of the country's largest mining operations continues today, pulling about 2 million tons of high-quality limestone from a huge mine between Davenport and Buffalo.
Some of the digging was done above ground — into cliffs or by quarry excavation. But many others dug deep into the earth, withdrawing massive amounts of coal for heat and power and limestone for Quad-City roads and buildings.
While many maps and other records exist, showing locations of abandoned mining operations, others never were properly marked or recorded.
And that is why we don't have a true and accurate picture of what lies beneath.
The Moline mine
As a teenager, Matt Cordes picked the one thing that would worry his dad the most.
He crawled into the old cave.
When his dad, Mike Cordes, was a teenager, he uncovered the lifeless limb of a neighbor boy who was buried by and suffocated in a cave collapse in Moline.
The elder Cordes ran to police that April day in 1963, and authorities uncovered the remains of Julius Dean Carpentier, 13, and Nicholas Rita, 12. The pair was doing something many children in the area of 34th Street had been doing for years — digging in the sandy clay of a 25-foot-tall hillside known as "swallow cliffs" for the birds that built nests into it.
"We just dug into the cliff, because the dirt was soft," Mike Cordes said. "We'd dig a pit and sit up in there like a little cabin."
When the parent of one of his son's friends told Mike Cordes that Matt was in one of the caves, he marched right out the door and headed straight for swallow cliff.
But he had the wrong place. His son, in fact, was some 30 feet underground — in an old limestone mine that few in the neighborhood knew existed, even though it was, literally, right under their feet.
Twenty years later, Matt Cordes remembers his adventures in the old mine as if they happened yesterday. He even recalls the moment he heard his dad's voice from above and scurried back to the opening, where a hand reached in and pulled him by his T-shirt to the surface.
And his dad has vivid memories of the giant cave, too. Once he realized his son had stumbled upon an old mine shaft and not a hand-dug pit, he wanted to see it, too.
Others followed, and their memories mingle with regret that they didn't spend more time exploring.
'I was like Huck Finn'
Even today, Matt Cordes gets nearly breathless when he remembers his adventures inside the massive Moline mine.
Now 37, Cordes was 16 years old when he and a buddy, Nathan Harvey, found an odd crevice in a hillside east of 34th Street. They shimmied down a tree root, curious to see where the opening led. At the bottom of a sloping 20-foot passage, a beach and a lake appeared, then a massive room — its ceiling some 20 feet above the water.
"I was like Huck Finn in 'Tom Sawyer,'" Cordes said. "You never forget something like that. I could write a book."
Cordes and his pals, Harvey and Jesse Essell, were creative in their pursuit of adventure.
To truly explore, they realized, they would need a boat. So, Cordes used the money he made doing roofing jobs with his dad and bought a blow-up raft at Kmart. The teens poked the boat through the opening, and once inside, they blew it up.
Cordes had the presence of mind to instruct one of the boys to stay ashore. The cave was immensely dark, and their flashlights were sufficient to light only what lay directly in front of them. The vast darkness diluted the beam of their flashlights, so one of the boys was assigned to remain on shore, shining his flashlight on the lake as a constant orientation.
"Directly inside, there was a Model T car frame, sticking out of the water," Cordes said. "We found bottles from the 1800s that crumbled in our hands.
"There was a series of large rooms, and I remember how cold it was. On a summer day, you could see your breath."
Between the time he discovered the breach in the shaft and the time the property owner ordered it resealed, Cordes estimates he went on a dozen expeditions.
"It isn't like the caves in Maquoketa," he said. "It's much bigger, and it's all connected."
When stories were written about the rediscovery of the cave 20 years ago, one man came forward to say a long-ago relative worked in the mine. He said teams of horses frequently were driven in and out, hauling wagons of limestone.
He also said he was told the mine stretched from 5th Avenue to Avenue of the Cities — 18 city blocks.
"We never got that far, because we didn't have good enough light," Cordes said. "It's a pretty amazing experience to have."
His dad agrees.
"I went in there after Matt started explaining that it was a mine shaft," Mike Cordes said. "The boys had tied a rope to the roots of the tree, so I went down there.
"It was one giant lake and cavern, and it had these stone pillars all around. In some places, I'd estimate the water was at least 10 feet deep, and you could see the bottom. It was another 15 or 20 feet to the cave's ceiling from the surface of the water.
"I was pretty eager for them to close that thing up. Kids are explorers."
More eyes in the mine
Before the owners of the property at Moline's 34th Street and 5th Avenue could hire a crew to reseal the old limestone mine, others were drawn in by the desire to explore.
Steve Etheridge was the Moline police chief in 1996, and his neighbor was Todd Rosenthal, long-time Moline High School wrestling coach and, at the time, a city alderman.
Matt Cordes was one of Rosenthal's wrestlers, and he let slip about an exciting discovery he and a friend made near their house.
Rosenthal told Etheridge, and the pair headed for the mine.
"I think about it all the time, because I drive by it all the time," Etheridge said. "I look to see if erosion has opened it back up. All I could think of was somebody getting hurt in there."
But that didn't stop him from making his own little expedition with Rosenthal.
"When you got through the opening, you landed on an area I call 'the beach,'" he said. "It was surprising how much you could see in that crystal-clear water — even frames from really old cars.
"It was a really huge, massive room. I would say the ceiling was 25 feet high, and the pillars were stone, which they'd just dug around."
Rosenthal has the same recollections and added, "It was really wide open; not claustrophobic.
"You definitely knew it was all man-made, and the ceiling was high and huge."
Despite the presence of the mine, contractors managed in 2010 to build a senior-living complex on property above it.
Neither the builder nor the operators of the Morning Star Senior Residences, 3601 6th Ave., responded to requests for comment on how construction may have been affected by the presence of the mine.
But Etheridge said he saw obvious signs of surface sinkholes.
"When we were inside the mine, you could see where loads of stuff was dumped to fill holes," the retired police chief said. "From above, you probably got the idea you were accomplishing something, but they weren't making a dent."
Hiding the hooch
Augustana College geology professor and department chairman Jeff Strasser has studied exposed limestone in Davenport's Lindsay Park.
He concluded, among other things, that limestone was mined from the area and used to build foundations for homes — probably in the vicinity of McClellan Heights.
"They were using it to pave roads, too," Strasser said. "The closer, the better, because they didn't have to haul it. In terms of natural resources, that was really important. Those little operations were probably pretty important."
Karen Anderson has spent more than 40 years in the Village of East Davenport and is a local historian and program director for the Antoine LeClaire House. In addition to what she has learned about the cavern that existed along Mound Street, north of River Drive, she has personal memories of it.
"When I was a kid, they called it the bears' cage because of the bars they put across the front," she said. "All under Davenport is riddled with these cave systems."
She keeps a file on caves, including accounts from people who built homes in McClellan Heights of the tunnels they discovered while digging.
While Strasser said the immediate Quad-Cities likely has no large natural cave systems, it is possible some of the smaller natural caves were made larger for specific purposes.
Anderson said the Village of East Davenport cavern is one such example.
"During a prohibitionist raid, 700 barrels of ale were destroyed — chopped open — in the bears' cage," she said. "The cave didn't come from quarrying. Caves in the area were used by early settlers, but they didn't have to dig them. They already were there naturally.
"It is likely they expanded on them during Prohibition."
The bears' cage was bulldozed in 1955, Anderson said. Ten years later, a collapse near 11th Street resulted in the dumping of 400 truckloads of sand to backfill the resulting sinkhole.
While historical records show the cavern at the Village of East Davenport was used as refrigerated storage for a local brewery, little other information is available.
Asked whether city workers have encountered any tell-tale signs of the bears' cage, Robbin Dunn, communications and preparedness manager for Davenport Public Works responded in an email: "A few of us have also heard of and seen old pictures of a cave/cavern north of River Drive at Mound (Street) thought to once be connected to a brewery, but none of us have any specific detail or information regarding its existence, other than what we have seen or heard."
In some areas of Rock Island County, homeowners still buy mine subsidence insurance.
While state geologic surveys show dozens of coal mines in the Quad-City area, not every mine was marked. Because coal was taxed as it was hauled above ground, some people kept their mines to themselves.
"There are a lot of them around still, and people keep finding them, and they're really hazardous," Strasser said. "It is an ongoing safety hazard."
As senior principal for Bettendorf-based engineering and science consulting firm Terracon, Ken Beck is one of the people most familiar with the Quad-Cities' underside. The company performs ground-penetrating radar, among other pre-construction services. Mining activity was commonplace throughout the area, he said.
"Every once in a while, one of those opens up," he said. "In the late 1980s, they had a problem by the hospital and water tower in Silvis. They kept throwing concrete in the hole and, each year, it got bigger.
"There was quite an extensive underground mine in that area."
Occasional sinkholes can be a nuisance to property owners, for sure, but the issues here with abandoned mines have been costly, not catastrophic.
"It really astonishes me: In all the 30-plus years I've worked here, we see the shafts pop open," Beck said. "But I've never seen anything that was the direct result of a subsided coal mine. The advantage is that most of these mines were pretty small. When they collapse, the soil does tend to build a bridge."
Most Quad-Citians are familiar with the local mine that is anything but small.
Linwood Mining, on Iowa 22 between Davenport and Buffalo, began operations in 1944 and went underground in the 1960s, according to Jonathan Wilmshurst, president of the operation. The family-owned business employs 100 people.
Once one of the nation's largest mines, it now would be regarded, simply, as large, Wilmshurst said. And that's because the company shifted its focus from massive amounts of limestone to a more chemical-based market.
"We're more interested in adding value to rock than chasing tons," he said, adding that the mine contains enough material to keep the company in business for several more decades.
The underground space at Linwood is massive. Vehicles drive in and out all day, and offices are maintained underground. The distance from the entrance to the mine face is 1.5 miles.
Other Linwood facts:
• The mine uses 100 percent of the methane produced by the Scott County Landfill, which occupies land above the mine. Combined with natural gas, the methane provides the "wick" for igniting solids in the lime kilns.
• Crystal from the mine is "highly prized" by rock collectors.
• Two ledges of high-calcium limestone are being mined at 75 and 120 feet underground.
• Most of the product removed from the mine, about 55 percent, is shipped by truck. Barges carry 35 percent, and 10 percent is shipped by rail.
When the limestone is all used up, portions of the mine that have not been backfilled could become public space, Wilmshurst said. Some old quarries are made into natural parks, he said, while others are used for cold storage. In Linwood's case, the mine is too damp, he said, to be of much use for storage.
But it's also possible people will be drawn to the area.
In some cases, he said, abandoned quarries become attractive to residential developers, because they offer lakeside living. A mile or more of underground mine, however, will require a creative approach when the time comes.
Meanwhile, it is the lesser-known caves and mines that interest adventurers.
"There were some small documented caves on the bluffs on the west end of Arsenal Island in the late 1800s, but I don't think they extended into the rock layer for any significant distance," Augustana's Strasser said. "As for large, natural caves, I'm pretty doubtful of their existence.
"Certainly, there are — and were — lots of small caverns, especially along some of the bluffs. If there was anything of value underground, whether it was coal or limestone, industrious people dug it out. People dug out whatever was there.
"As time goes on and residential and commercial developments expand, it is likely more of these places will be discovered. Some of it will just stay what it was — tunnels we can't see."