BURR OAK - Gloves grip cold, gritty rungs as boots thud against lower ones.

A knee bends and the body maneuvers a notch deeper down the 95-foot ladder chute.

The air chills and echoes of surface conversation fade against the sound of rushing water.

Welcome to Coldwater Cave.

The 17-mile cave, just south of the stateline beneath the hilly corn and soybean fields of Winneshiek County, is the longest, most significant cave in the Upper Midwest.

The cavern is closed to the public, but owners of two manmade entrances offer access to scientists and surveyors who are working to unlock the cave's mysteries.

Cave enthusiasts from Iowa, Illinois and Wisconsin meet on the third weekend of each month to collect data, map passages and hopefully protect the natural landmark for future generations.

"It's not an adventure or thrill seeking," said Mike Lace, coordinator of the Coldwater Cave Project. "The work we do in the cave is rewarding in that we come to have a better understanding of how the cave forms and functions."

The cave is contained in a section of landscape known as karst, which makes up much of Northeast Iowa. The terrain is known for its many springs, sinkholes and limestone caves. None are as spectacular as Coldwater Cave.

A pair of scuba divers discovered the cave in the late 1960s by swimming into Coldwater Spring. The spring is the cave's only natural entrance, where a frigid stream that runs through it emerges at the base of a 150-foot cliff.

Upon hearing of the discovery, the state - interested in the prospects for commercializing the cave - took out a four-year lease on a portion of nearby farmland and drilled the first artificial entrance. When the lease expired in 1975, though, it decided to abandon the effort and it reverted back to the landowners, Ken and Wanda Flatland.

The Flatlands have for 30 years welcomed volunteers to their property to study and care for the cave, which was named a National Natural Landmark by the U.S. Department of Interior in 1987. A second entrance was created in 2003 by another landowner, John Ackerman, who also has sought to accommodate research projects in the cave.

At the Flatland entrance, volunteers pull themselves into wetsuits inside a metal and plywood bunker that covers the entrance. Helmets cap skulls, and double-thick rubber socks wrap under heavy-duty hiking boots. A shiver still strikes as boots first clomp into the rocky, rippling stream.

Necks arch as headlamps illuminate clusters of delicate stalactites and soda straws, which dangle wherever cracks split the ceiling. Formations grow over centuries as rainwater seeps into the cave, slowly depositing minerals where it waits to drip.

Sloshing downstream, walls are glazed with flowstone - mineral deposits left over by water trickling across the surface. The scene looks like a terrible candy factory mishap, with carmel, marshmallow and dark chocolate patterns coating walls.

The day's mission in the cave is part of an ongoing effort to better understand the cave's hydrology, or how water flows in and out of the system. This might help determine how water and pollutants move throughout the entire watershed.

Science coordinator John Lovaas and project volunteer Dawn Ryan, both of Woodstock, Ill., measure the channel's width and depth. Next, they begin timing how long it takes an orange to float through a marked section of the stream.

"One, two, three… " Lovaas counts, signaling for Ryan to release the fruit, which bobs through a current across an imaginary line where Lovaas clicks a stopwatch and calls the time.

Another caver catches the orange and the drill is repeated several times for accuracy. The measurements allow for estimating the cubic feet of water flowing through the cave per minute.

Further downstream, Lovaas uploads data from one of a half dozen temperature readers kept in the cave. The devices detect water temperature every 10 minutes and can store up to six months worth of data. Because the air temperature is consistent in the cave, fluctuations suggest an inflow of cooler or warmer water from the surface moving through the passage.

Patricia Kambesis, an environmental researcher at Western Kentucky University, has studied the watershed around Coldwater Cave for half a decade. The research is finding the same porous limestone topography ideal for cave formation also makes the territory vulnerable to pollution from chemical and manure spills on the surface.

A recent series of experiments involved spilling liquid dye at points near the cave and tracking where and how long it takes to show up in the cave. In non-karst areas, surface water can take days, weeks, months or years to sink into the groundwater. Around Coldwater Cave the dye trace spills entered the groundwater within hours.

"It's a far more complex system than people previously assumed," Lovaas said.

When county supervisors considered a request last summer for a 4,800 unit hog confinement operation, Kambesis submitted a summary of findings that explained the threat a hog confinement could pose to water quality in the area.

The cave's water flows to the Upper Iowa River, which could see increased nitrate and bacteria levels, Kambesis said. That could affect drinking water for Decorah, which gets some water from sand and gravel aquifers associated with the river.

While the cave research suggests the fragility of the watershed, the cave itself is also delicate. Cavers in the project are careful to avoid touching formations and even abandon exploration where formations would have to be disturbed.

"It's still a very pristine environment despite 30 years of activity in it," Lace said.

Lovaas said caves are likely the least studied and least protected of all ecosystems, which makes stewardship critical. It's "the last bit of wilderness in the Midwest."

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Contact Dan Haugen at (319) 291-1565 or dan.haugen@wcfcourier.com.