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Arizona State Parks recently celebrated the 10-year anniversary of visitor access to Kartchner Caverns' Big Room, which is closed during part of the year because it is inhabited by bats. (Arizona State Parks Photo)

BENSON, Ariz. -- Deep inside the dark, damp caves of Kartchner Caverns State Park lies something that sounds more suited to a fairy tale than to a rock formation.

The so-called Big Room holds the world's largest formation of brushite moonmilk -- sometimes called "elf's milk" -- a sparkling white, creamy-looking substance that occurs when bat guano mixes with limestone.

The moonmilk might be nicer to look at, but it's the bat poop that really makes the cave special, said Robert Casavant, natural resource and science manager for Arizona State Parks.

"The bat guano is the basis of the Big Room's ecosystem," he said.

For 10 years, the Big Room has been open to visitors from mid-October through mid-April, when its resident bats migrate elsewhere, complementing the Throne Room that opened to visitors in 1999.

Arizona State Parks celebrated the Big Room's anniversary this month with special speakers and presentations, including the dedication of the Kartchner Caverns Underground Microbiology Exhibit.

The new exhibit focuses on fungi and bacteria that live in the caves.

Because of the bat guano, the Big Room has the highest concentration of these pinhead-sized creatures, but they are part of what makes this cave so special, Casavant said.

The Big Room is the first part of Kartchner explored by Gary Tenen and Randy Tufts, who discovered the caverns in 1974 beneath the Whetstone Mountains. They kept them secret for 14 years before beginning a process with the landowner and officials that led to the state park.

The mainstay of the park is the multiple Throne Room tours that are given every day, but the added revenue of the Big Room tours six months of the year has been a financial boost for Kartchner, especially when State Parks faced tough times during the recession, park ranger Rachel Thompson said.

"We faced our lows in 2009 and 2010 with weekly furloughs," Thompson said.

After an eight-year low of 119,000 visitors in fiscal 2011, the park's attendance has been steadily rising, seeing 129,000 visitors in fiscal 2012 and almost 132,000 for the year ending June 30.

Volunteer tour guide Dave Ottens said, "We enjoy our state park and it makes us happy when visitors come here and enjoy it, too."

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