Matt Gentry | The Roanoke Times

A flat-bottom boat sits in the mud flats of Mountain Lake. The lake level is 57 feet below "full pond" as of Aug. 12.

Matt Gentry | The Roanoke Times

Patterns have formed in the encrusted mud flats of Mountain Lake, which is one of only two natural lakes in Virginia. The other one is Lake Drummond in the Great Dismal Swamp.

Matt Gentry | The Roanoke Times

A recreational pontoon sits at the end of a temporary boat dock Friday at Mountain Lake in Giles County. Biologists from Roanoke College and Virginia Tech believe that a fault in the mountain and a fissure at the bottom of the lake contribute to the fact that it drains on a cyclical basis.

"What lake?"

That is how longtime Mountain Lake Hotel General Manager Buzz Scanland responds to questions about the condition of Giles County's popular resort attraction.

"Nobody living's seen it this low," Scanland said Friday.

The level of the 50-acre natural lake showcased in the 1987 hit movie "Dirty Dancing" has been dropping since 2006 and today stands at about 5 acres, Scanland said.

Where visitors have for generations swum, boated and fished, a meadow is quickly sprouting. Only a shallow pond surrounded by knee-deep mud remains. As the water drains, so does the hotel's occupancy rate. July and August are typically the resort's busiest months, but business is running 15 percent to 20 percent below normal, the general manager said.

This is the second time Scanland has faced this problem. In 2002, the water dropped so low that the board of the Mary Moody Northen Endowment -- the Texas-based organization that owns the resort -- drilled a test well that it was hoped might produce enough water to refill the lake.

The effort failed. But by 2003, conditions shifted and the lake refilled on its own.

Two years ago, the level again began to drop. Today, children staying at the hotel search the drying lake bed for "treasure" -- old bottles and other artifacts once lost in the water, now exposed to the sun.

The cause of the drain is not as simple as drought or even climate change. In fact, it's cyclical.

Mountain Lake may be the only known lake in the world to periodically disappear, sometimes for decades. This phenomenon has been recorded since the 18th century and may be related to earthquakes.

The first reference to what is now called Mountain Lake dates to 1751, when British surveyor Christopher Gist explored Giles County and West Virginia for the Ohio Company. In his journals, Gist describes climbing a mountain and finding a clear-water lake, a gravel shore and "fine meadow." By 1768, however, settlers said such a lake was nowhere to be found in what is now Giles County. Instead, they used the site as a salt lick for cattle, hence the name Salt Pond Mountain.

Biologists Jon Cawley of Roanoke College and Bruce Parker, who is retired from Virginia Tech, have plumbed the lake bottom and even farther below to discover the secrets of its hydrological idiosyncrasies. In his analysis, Cawley said that water ran down a fault in the mountain and eroded the softer rock, creating a basin that filled with water more than 6,000 years ago.

That fault is also at the center of Cawley's theory on the lake's periodic disappearance. A large fissure in the lake bottom sits directly on the fault. After plunging to a depth of nearly 110 feet, the hole becomes a pipeline through the bedrock that Cawley and Parker believe eventually resurfaces about half a mile downstream.

Normally, water flows into the lake from five feeder streams and via rain and snowmelt at roughly the same rate at which it escapes through evaporation and its natural drainage routes. But during dry years, water continues to leak out through the bottom hole. The biologists have found historical records of rising lake levels following earthquakes on the mountain in 1899 and 1959.

There's no way to predict how long this latest dry spell will last. Staff at the Mountain Lake Conservancy -- the nonprofit group that oversees recreation and other activities -- are working to create more attractions for visitors. The group puts on the annual Dirty Dawg Cycling Weekend, the BrewRidge Music Festival and the Mountain Lake Migratory Birding Festival. They also offer spelunking and rock climbing, among other programs and activities, said Emily Woodall, managing director.

But the biggest attraction these days is the legacy of "Dirty Dancing." According to Scanland, fans of the British reality television show "Dirty Dancing: The Time of Your Life," which has shot two seasons on location in Pembroke, have been flocking to the hotel. The U.S. tour of "Dirty Dancing: The Classic Story On Stage" also has stoked interest.

"It just seems to never stop," Scanland said of the popularity of the movie and its spinoffs. "As long as the lake's down, it's going to be as big a draw as we can make it."

On the Net: www.mtnlakeconservancy.org