Travelers know the warning: Don't drink the water.

But tourists to Mammoth Cave National Park might be better off learning a new one: Don't let the water drip on you.

National Park Service employees and Western Kentucky University researchers are working on finding out why water at Mammoth Cave has shown spikes in fecal coliform and E. coli bacteria.

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To protect visitors, park workers have installed a plexiglass and steel cover at the cave's historic entrance where dripping water naturally finds its way downhill and onto visitors to the world's longest cave.

Part of the historic section of the cave was closed in late October after a WKU researcher found the presence of the contaminants at the cave's waterfall, Charons Cascade. The historic section was reopened in mid-November after the shelter was installed.

"The first thing I need to say is that the historic section of the cave is open and safe," Superintendent Patrick Reed said in a statement. "We still don't have all the answers regarding the spikes in E. coli that we have found recently in the cave water."

Park officials stress that no one has become ill, especially park workers who are warned to wash their hands every time they leave the cave, said Vickie Carson, a spokeswoman.

Park officials at first thought that the spikes in the contaminants might be related to heavy rainfall flushing the naturally occurring bacteria toward the cave, Carson said. That apparently isn't the case.

"It's going to take a while to figure out what's going on and what we have to do to mitigate it," Carson said.

While park officials don't know if the E. coli strain is from humans or animals, they don't think the source is coming from outside the park, Carson said.

They've even checked the park's sewer system for leaks. All of the sensors show that the park's sewer lines are not the cause, Carson said.

A team is monitoring water samples four times a week in the cave to test for the bacteria.

Bob Ward, who is chief of the park's cultural resources, is heading the team investigating contamination. He said the schedule for testing varies from week to week, but employees collect 50 milliliters of water for each sample and use 10 milliliters to test for E. coli. The rest is discarded.

The testing has required a few changes in tour schedules, and one section of cave — Mammoth Dome — is still closed to the general public, Ward said. Park officials are considering options to reopen it, including another plexiglass cover, he said.

Carson said most of the upper portions of the historic cave are completely dry so that's where tours are being taken. The cave has several sections where contamination has not been a factor.

The cave is nestled in the Green River valley and hill country of south-central Kentucky, with more than 365 miles explored since 1816.

Attendance has not been affected by the changes, Carson said, adding that visits over the Thanksgiving weekend were up compared to last year.

Ward said the park has tried to be up front about what's happening. It has posted signs throughout the park and tour guides also inform visitors about the situation.

Park officials are operating under federal health standards that typically apply to the public engaging in activities in water like swimming and water skiing.

"Here, people feel the dripping water, look up and can get drips in their mouth, nose or eyes," Carson said. "We are being cautious."