It's difficult to imagine an animal less likely to draw admiring crowds at the Maryland Zoo in Baltimore.

A homely giant salamander that hides under rocks, slimes its enemies when threatened, and goes by such aliases as snot otter, devil dog and Allegheny alligator, the hellbender is nevertheless seen as an important and valuable addition to the zoo's collection.

Two of the increasingly scarce animals inhabit Hellbender Country, a $200,000 exhibit that is set to open Thursday as part of the Maryland Wilderness area.

The animals immediately began exploring the faux rocks and sheltered crevices at the bottom of their simulated mountain stream Tuesday, when assistant curator Kevin Murphy introduced them. One curiously inspected a photographer's underwater video camera until the exhibit's bright "daytime" lights finally sent the nocturnal creatures into a corner for a snooze.

Zoo officials hope the hellbenders and the Baltimore exhibit become a more active part of Head Start, an effort led by zoos in Buffalo, N.Y., Wheeling, W.Va. and St. Louis, Mo., to educate the public about the salamanders, and to hold, raise and return hundreds of them to their native streams. In time, Murphy hopes the Maryland Zoo staff may learn to breed them.

"A huge component of the Maryland Zoo is the Maryland Wilderness," he said. "And a really big component is to work with our local species. The hellbender is arguably Maryland's most endangered animal species."

Besides, he added, "most people haven't heard about hellbenders … and they've got a really cool name."

Known to science as Cryptobranchus alleganiensis, hellbenders are North America's largest salamanders, and the third biggest in the world.

About 11 to 20 inches long and 3 to 5 pounds as adults, the Eastern hellbender can live up to 30 years in the wild or in zoos. Their range today includes mostly Appalachian mountain streams from northern Alabama to New York State, where they dine on crayfish, earthworms and insects.

But they are in trouble almost everywhere. Listed as endangered in Maryland, they survive in just two remaining habitats, in far western Garrett County.

"Historically, they were also in the Susquehanna [River] system, in Deer Creek and Rock Run in Cecil and Harford counties. But we haven't had a confirmed positive sighting there for decades," said Scott Smith, a wildlife ecologist with the Maryland Department of Natural Resources.

In Western Maryland, the hellbenders' range and numbers have been reduced by acid mine drainage, siltation caused by logging, and collectors, Smith said.

"There are people who are loving them to death, and some are selling them into the pet trade," he said. "It really can't sustain collecting pressure."

Just searching for the hellbenders can ruin critical habitat. Their well-being requires swift, clear, cold streams littered with jumbles of flat rocks washed out of the stream banks. Moving those rocks to look for hellbenders can demolish the sheltered spaces beneath, where the animals hide and raise their young.

And, since hellbenders "breathe" through their skin, they are particularly vulnerable to water-borne contaminants.

"It's a species that many people believe should be federally listed [as endangered], and I'm one of them," Smith said. "It's about the closest thing we have to the primordial amphibian that first evolved. It's … extremely primitive, and we're fortunate it's one of our native species. And it's really hanging on by its claws."

The Maryland Zoo's exhibit was designed to mimic the hellbenders' native mountain streams. The tank is 10 feet long by 2 feet deep, with 18 inches of moving fresh water that's to be kept at 70 degrees in summer, and 42 degrees in winter.

It was constructed by Peeling Productions, in Allenwood, Pa., from molds of stream rocks in that state's Little Pine Creek, and installed in a cave-like room beneath the otter pool.

On Tuesday, barely 20 minutes after the hellbenders were introduced to their new home, they were alternately exploring the rock crevices and hiding out. Although they have vestigial lungs, they live their entire lives under water, crawling fluidly among the rocks, or sitting motionless waiting for a meal to happen by. Their mottled brown and gray coloring allows them to blend in with the stream bottom.