Peter O'Shea, a retired police sergeant, has spent 23 years searching the forest trails, sand deposits and snowdrifts of the Adirondack Mountains for cougars. He has seen a grand total of none.

That could mean one of two things.

Perhaps cougars are not here to be found. That is the position taken by government managers -- that the cougar subspecies that once roamed these mountains and the rest of the East Coast has probably been extinct for decades.

O'Shea prefers to think the cougars are just really, really good at hiding.

"Cats, you know . . .," he said. "There's nothing more furtive than a feline."

This is the kind of attitude -- persistence that runs to the edge of absurdity -- needed to look for one of nature's ghosts. It is a common trait among the small group of scientists and environmentalists who are still searching for the 27 creatures, including the eastern cougar, that are classified as "presumed extinct" by the U.S. Fish and Wildlife Service.

To find them, they have braved 20-inch Hawaiian rainfalls, groped along the bottoms of coffee-colored rivers and spent months vainly broadcasting bird songs through snake-filled swamps. They have faced all the tedium of normal biological research, plus the added burden of not knowing if it is all a gigantic waste of time.

This year, one success -- the rediscovery of the presumed-extinct ivory-billed woodpecker in Arkansas -- has improved the spirits of this eclectic group. Maybe, some think, there is more good news out there in the woods.

"We have not given up," said Eric Vanderwerf, a Fish and Wildlife Service researcher in Hawaii who is looking for nine vanished bird species.

The 27 species on this list are the most hopeless among the 394 U.S. animals now classified as endangered, but the government is not quite ready to write them off. In most cases, it was humans that put them in their dire straits -- a startling measure of our impact on the environment, since their habitats are as diverse as the island of Molokai in Hawaii and a single riffle of a single stream in central Ohio.

Some of the damage was done years ago: Hawaiian birds were decimated by a disease-carrying mosquito, which may have arrived on the first European ship to visit the island in the 1700s. The Eskimo curlew, a bird that once numbered in the hundreds of thousands on western prairies, declined sharply during a wave of hunting in the late 1800s.

Other problems are much more recent. Scientists in the Southeast blame dams and federal water projects built as recently as the 1980s, which slowed the flow of rivers. That killed off small freshwater mussels that had evolved to depend on a fast current.

"The really frustrating thing is that we're losing fauna in our lifetimes," said Paul Hartfield of the Fish and Wildlife Service office in Jackson, Miss.

These scientists say that, while their efforts might be spent on animals with a better chance of survival, they feel a kind of moral obligation to the presumed extinct. Because humans' indifference drove them to their current condition, the scientists reason, these animals deserve every effort that might resurrect them.

Hartfield explained it by paraphrasing a quote from biologist E.O. Wilson, a leading expert on biodiversity.

"The first rule in good tinkering," Hartfield said, "is 'Save all the parts.' "

For his research, the "parts" in question are species such as the southern acornshell and the black clubshell, shellfish that live (or lived) in the gravelly bottoms of southeastern rivers.

To find them, some scientists wind up snorkeling or scuba diving with their faces a few inches off the bottom, keeping an eye out for snapping turtles and alligators as they search. And that is if they are lucky.

The unlucky ones have to dive in rivers now so choked with dirt that there is not even a few inches of visibility. They have to feel their way -- using touch to determine what is a piece of gravel and what is a tiny shellfish the size of a piece of gravel.

"It's kind of like caving, without a light," Hartfield said.

Things are different, but not really much better, for the scientists who look for Hawaiian birds such as the large Kauai thrush, whose only known habitat is near the peak of Mount Waialeale, which just happens to be the rainiest spot on Earth, with 500 inches a year.

"It's almost a constant drizzle," Vanderwerf said. The undergrowth grows so quickly that they find themselves bushwhacking through what seems like a solid green wall.

In the Southeast, scientists seeking the Bachman's warbler -- a swamp-dwelling songbird last seen in 1962 -- say one serious enemy to success is tedium.

That's because this is how you look for a Bachman's warbler: Walk 400 feet through a swamp. Stop. Play a recording of the male bird's song. Then listen for a response from a bird out in the woods -- perhaps one of the bird's distinctive calls that the literature describes as a "zeep or a buzzy zip."

Then, if you don't hear anything, repeat the process.

For how long?

"Just days and days and days," said Paul B. Hamel, a biologist with the U.S. Forest Service in Stoneville, Miss. He spent two months every year doing this from 1975 to 1979, without seeing a single warbler.

These scientists are heartened by the story of the ivory-billed woodpecker. This large bird with white wing patches had not been seen since 1944 despite intensive efforts. Then a persistent searcher spotted it in a swamp this February.

That startling find gave a special boost to the amateur biologists looking for the eastern cougar, considered to be a particularly hopeless case.

"If that bird could disappear for 50 years and still be present, well, gosh, that makes it much more credible that the eastern cougar could still be there," said Chris Bolgiano, vice president of the West Virginia-based Eastern Cougar Foundation.

The cougars, close relatives to Florida panthers and mountain lions of the western states, used to roam from Florida to Maine -- including the mid-Atlantic forests that would become the District and its suburbs. But they were hunted for bounties and lost their main prey as deer were displaced by agriculture.

The last eastern cougars are thought to have lived in the Great Smoky Mountains in the 1970s. Any mountain lions found in the East since then -- and there have been a few -- have been classified as escaped zoo animals or pets.

But some still believe the native cats hung on in the remotest areas, and they point to scattered sightings and tracks as evidence.

Final proof, however, remains elusive. Take the case of Noah Charney, who had been hunting for cougars in western Massachusetts recently when he found a deposit of animal waste that bore all the marks of a cougar dropping.

He sent it off to a lab, believing this might at last be the missing link.

"It was just a giant bobcat," he said disappointedly.

"You're certainly riding this 'Oh, we're about to get it . . .,' " Charney said. "And then, 'Aw, no.' "

In the Adirondacks, O'Shea said he has seen five different sets of cougar tracks in the snow over the years, with the drag marks left by a long tail signifying that the tracks were not left by bobcats or lynx.

He feels he's due for a real sighting -- and that the East is due to have its cougars back, actually and officially.

"If they aren't present," he said, "the ecosystem isn't whole."

Peter O'Shea at one of the Adirondacks sites where he has spent 23 years searching for the eastern cougar.