May 27th 2007 Death Valley National Park -- The last place anyone would expect to find fish is Devil's Hole, a chasm in the middle of the Mojave Desert where a 100-degree day is mild and the only thing bigger than the rocky expanse of desert is the sky above it.

Desert pupfish in hot water, Only 42 left: Creature whose plight led to the Endangered Species Act is on the brink -- researchers don't know why.

A team of divers prepares to submerge in Devil's Hole as lead diver Mal Maloney gives a thumbs-up. Chronicle photo by Kat Wade

But nature is nothing if not amazing -- as good an explanation as any of how the Devil's Hole pupfish has survived in the bottomless geothermal pool that gave the fish its name. It is tiny, just an inch long, yet few species loom so large in the history of American environmentalism.

The Devil's Hole pupfish is one of the rarest animals in the world. The seemingly endless effort to save it laid the foundation for the Endangered Species Act and shaped Western water policy a generation ago with a landmark Supreme Court ruling.

But after 20,000 years in the desert, the fish teeters on the edge of extinction. No more than 42 remain in Devil's Hole.

The Devil's Hole pupfish has been the beneficiary of one of the most aggressive campaigns ever to preserve a species, an effort every bit as intense as those to save the bald eagle and California condor. The Endangered Species Act requires nothing less. But saving the pupfish is more than a legal obligation for the biologists and bureaucrats involved.

It's a moral one. "This fish is the species that made us take note of our need for conservation," said Mike Bower, a National Park Service fish biologist. "It made us realize that our actions have an impact beyond us. We have a responsibility to look after this fish."

No one knows why they are vanishing. No one knows what it might say about the health of the desert. And no one knows whether they can be saved.

More than the loss of a species is at stake at Devil's Hole. A deeper question has been posed in the desert outside Las Vegas, where scientists have spent the better part of 60 years trying to keep the pupfish alive: Should we even bother? Or are we only delaying the inevitable?

Devil's Hole is just that -- a hole on the side of a hill overlooking an oasis called Ash Meadows. It's about the size of a mineshaft and looks about as interesting.

But beneath the surface lies a limestone labyrinth filled with crystal-clear water that fell as rain eons ago. A diver once descended to 450 feet, and researchers once sent a camera 100 feet beyond that. It keeps going from there. How far is anyone's guess.

Yet the pupfish spend most of their time foraging and spawning on a rock shelf just below the surface of the water. They live in almost complete isolation in alkaline, 93-degree water that contains very little oxygen. They feed on algae, snails and other tiny invertebrates in what is one of the world's smallest ecosystems.

The Devil's Hole pupfish is the oldest of the seven pupfish species -- each named for where it's found -- remaining in Death Valley, but no one knew it was there until the 1890s. Forty years passed before biologist Joseph Wales realized it was a unique species related to, but different from, the others.

Devil's Hole pupfish have larger heads and slimmer bodies than their cousins in Ash Meadows and elsewhere, and they lack the pelvic fins and stripes the others have. Males are bright blue. Females are a yellowish shade of brown. They dart about like puppies, hence the name, and if any fish can be said to have a personality, it's the pupfish.

"It's the best example of charismatic microfauna," said Jim Deacon, a retired University of Nevada at Las Vegas biology professor who has spent more than 40 years studying the animal. "It's so darn cute. It's a beautiful fish."

There have never been more than a few hundred pupfish in Devil's Hole, or a time when scientists didn't fear losing them.

The campaign to save the fish began in the 1940s when two biologists suggested making Devil's Hole a part of Death Valley National Park. President Harry Truman did just that in 1952. A locked fence was erected about a decade later after two teenage divers got lost in the labyrinth and drowned. Their bodies were never found.

As Nevada grew and demand for water mounted, so did concern for the pupfish. It was among the first species protected by the Endangered Species Protection Act of 1966.

Concern turned to panic in the late 1960s when a landowner started pumping water from the aquifer that feeds Devil's Hole. The water level dropped precipitously, threatening to expose the rock shelf essential to the survival of the pupfish.

As the water level fell, so did the number of pupfish. By 1972, only 124 remained.

The U.S. Fish and Wildlife Service moved a handful to a tank near Hoover Dam to prevent losing the species entirely. Conservationists sued to stop pumping from the aquifer.

President Richard Nixon signed the Endangered Species Act of 1973, expanding the protection of listed species to include "the ecosystems upon which they depend." And the government created a task force to save the pupfish.

In 1976, the Supreme Court unanimously sided with conservationists -- and validated the Endangered Species Act -- when it ruled that water could be drawn from Amargosa Desert Ground Water Basin, but the Devil's Hole pupfish is entitled to the water it needs to survive.

The water level in Devil's Hole, Chief Justice Warren Burger wrote, must not fall more than 2.7 feet below a copper washer -- since replaced by an ordinary bolt -- the government had affixed to the cave wall in 1962.

The rock shelf has remained submerged beneath an average of 15.7 inches of water ever since.

"The pupfish is largely responsible for forcing us in Nevada to use our groundwater responsibly," Deacon said.

The impact was immediate.

As the water level rose, so did the number of pupfish. It averaged 324 through the mid-1990s.

Divers have counted the pupfish, one by one, each spring and fall since 1970. Winters are tough because food is scarce, so there are always more pupfish in autumn. The annual population is an average of the spring and fall counts.

The fall population has topped 500 on a few occasions and hit 582 -- an all-time high -- in 1994.

At long last, it looked like the pupfish might make it.

Then they began to disappear.

It started in 1996. No one worried at first because the decline wasn't statistically significant. But the average population kept falling. 295. 265. 235. When the average population hit 180 in 2001, the biologists got desperate. No one could explain it.

Pupfish have no predators. There was no evidence of an invasion by a nonnative species. No sign the water had been polluted. Nothing to suggest the ecosystem had been degraded in any way. Everyone was stumped.

Experts have a few theories.

Rain washes dirt and gravel onto the rock shelf and seismic activity settles it, creating nooks and crannies that provide shelter for pupfish larvae and harbor the algae pupfish eat. Some believe the infrastructure -- the fence around Devil's Hole, a hydrometer that provides a constant measure of its water level, the platform used to observe the fish -- has somehow altered that process.

Others wonder if the population has grown so small that inbreeding has caused "genetic meltdown" and survival is no longer viable.

And a few experts suggest we might be seeing natural ebb in the development of a species that has evolved over 20,000 years.

"We really don't know how much their numbers fluctuate," said Jon Sjöberg, a biologist with the Nevada Department of Wildlife. "We might be looking at one small part of a big sine wave."

Nature and bad luck have conspired against the biologists.

A flash flood in September 2004 washed research equipment into Devil's Hole, killing 80 pupfish -- one-third of the population at the time. Pupfish living in tanks called refugia proved difficult to breed. Others were inadvertently mixed with their cousins from Ash Meadows, creating a hybrid species.

"It's two steps forward, then one step back, then someone whacks you in the head with a two-by-four," Sjöberg said.

Just 85 pupfish remained in Devil's Hole last fall. To minimize winter mortality, biologists started stocking Devil's Hole with food developed for the endangered silvery minnow.

They think it helped. Researchers counted as many as 42 pupfish during the three dives they made one day last month. That's up from 38 the previous spring.

More encouraging, the divers saw pupfish in a wide range of sizes, indicating there is a good mix of juveniles and adults. And six -- including three born this spring -- held in refugia are doing well.

"We're in a much better position this year than we were last," said Bob Williams of the U.S. Fish and Wildlife Service, which has spent about $750,000 on the pupfish effort since January 2006. "I'm optimistic. I think we can make great strides this year."

Pupfish spawn year-round but are busiest in April and May. Biologists expect the population to rise through the summer and hope to see at least 80 pupfish in the fall. Should they reach that number, they'll consider transferring at least eight to refugia to launch another breeding program.

Moving some pupfish eggs to refugia to improve their odds of hatching is another idea under consideration.

The biologists also may try breeding some of the hybrids with captives from Devil's Hole, then breed those offspring with Devil's Hole pupfish and so on for several generations. The idea is they will eventually return to a purebred Devil's Hole pupfish.

The biologists know saving the pupfish is a longshot. But they believe they must try.

It's important, they said, because the pupfish provides a glimpse of how the planet came to be. There are 119 species of pupfish in the world, all of them descendants of a fish that swam 200 million years ago when the continents were one.

As that land mass, called Pangea, broke up, pupfish spread throughout the world. Following the fossil records of those fish back through time offers clues to how the continents formed.

The Devil's Hole pupfish, though, is an enigma. No one knows how it got there. Solving that riddle could provide new insight into the geological history of the American Southwest.

Perhaps more important, the Devil's Hole pupfish might be trying to tell us something about the health of the 20 or so other species of desert fish in California and Nevada. Or it might be warning us of a mounting problem within the desert, or the water that sustains it, that we can't yet see.

Of course, it's possible the decline doesn't mean anything at all.

Nature is nothing if not amazing, but it can be brutal, too. After so many years in one of the least hospitable places a fish could possibly live, perhaps the Devil's Hole pupfish has simply reached the end of the line.

The biologists working to save it don't accept that.

"This fish has survived for 20,000 years," said Deacon, the retired professor who's spent half a lifetime studying the pupfish. "Its time hasn't come unless we decide that its time has come."