A few months ago, on a sunny afternoon in northern Vietnam, I set off in a small fishing boat outfitted with a motorbike engine to find the world’s most endangered turtle. Of the planet’s four known remaining Yangtze giant softshells (Rafetus swinhoei), only two reside in the wild. Both occupy separate lakes in northern Vietnam. The other two, the world’s sole surviving couple, live in a zoo in southern China; for years, scientists have been trying to breed them, unsuccessfully. (The male has a damaged penis.) The two wild turtles represent the final hope for the species. I had boarded the boat hoping to glimpse one of them in Dong Mo, a fourteen-hundred-hectare lake an hour or so west of Hanoi. I was joined by a group of Vietnamese conservationists allied with the Asian Turtle Program (A.T.P.) of Indo-Myanmar Conservation, a U.K.-registered nonprofit organization. The odds were against us.

The Yangtze giant softshell occupies a regal position in national lore. Legend has it that, in the fifteenth century, in a Hanoi lake called Hoan Kiem, the turtle was the keeper of a mythic sword used to liberate the country from Chinese invaders. Sightings are said to bring good fortune. In 2016, when Hoan Kiem’s sole softshell—which was nicknamed Cu Rua, or “great-grandfather turtle” in Vietnamese—was found dead, it made international news. (Politically, it was bad timing: some joked that its death, which came on the eve of an important gathering of the Vietnamese Communist Party, signalled the regime’s demise.) The country mourned. “It was like losing part of our culture,” Hoang Van Ha, an A.T.P. conservationist, told me.

The turtle in Dong Mo is estimated to be forty or fifty years old, which is considered quite young. For the past eleven years, A.T.P. volunteers have been doing near-daily patrols on the lake, keeping an eye on the turtle and observing its behavioral patterns. The species lives up to its name, with individuals growing to more than three hundred and sixty pounds. Though it used to be common in the region, a combination of hunting, environmental degradation, and rampant development has brought it to the existential brink. Dong Mo’s patrollers usually witness the reptile only once every few weeks, Ha told me, but observers had seen it surface several times in the hours before I arrived. It was the first clear day in weeks, and the turtle was basking.

Dong Mo is a soupy-brown color, and the lake is filled with bamboo-choked isles. It was formed in the seventies, when a dam was built on the Red River, the major waterway of northern Vietnam. Scientists think that the Dong Mo turtle was trapped there after the river’s damming. (Historically, the turtle has lived in large rivers and channels.) Conservation-wise, this was a blessing. “If this animal were out in the Red River, there would be nothing we could do to monitor it or give it protection,” Tim McCormack, an A.T.P. program coördinator, told me. In 2008, heavy rains caused the dam to overflow. The turtle escaped, but only into a fisherman’s net. It took six hours of negotiation between N.G.O. volunteers, the police, local cadres, and the fisherman to rescue the animal. In 2013, conservationists successfully lobbied for a law that fully protects the turtle from capture or sale.

Since then, other safeguards have been installed. On Dong Mo, the A.T.P. conservationists showed me a mesh net, sixty-five feet deep, that was laid across a portion of the lake to prevent escapes. I asked if they ever worried that the turtle would become entangled. “Normally, when he encounters the net, it just swims away,” Ha said. The conservationists lifted the net out of the water and tossed caught branches to the side. Nguyen Tai Thang, a conservationist wearing a pith helmet, repaired a hole with some rope. The turtle was nowhere in sight.

Though the giant softshell’s future appears grim, there have been a few inspiring developments of late. In April, the existence of the second wild specimen was confirmed in a lake called Xuan Khanh. The discovery was made using a new method known as environmental DNA, or eDNA, which reveals the presence of animals by genetically analyzing water samples. eDNA could be used to confirm other softshell specimens across Asia.

Genetics are also coming in handy elsewhere. Last year, a paper published in Scientific Reports showed that, contrary to prior understanding, softshell turtles possess sex chromosomes, which means that they can be sexed using DNA analysis. Minh Le, an evolutionary biologist at Vietnam National University and a research associate at the American Museum of Natural History, told me that preparations were already being made for a test on the Dong Mo turtle. (DNA samples were procured during its escape and capture in 2008.) Assuming things go smoothly, they should have results early next year.

The sex of the two Vietnamese turtles will determine the next course of action. If both are male, plans may be made to transfer either a whole turtle or semen samples to the Suzhou Zoo, in southern China, where the only known female lives. In July, the Suzhou Zoo made a formal appeal to the Vietnamese government to do just this, but no decision has been made. Since 2008, the zoo has been trying to get its pair to produce offspring, but the male’s member, mutilated in a battle with another male some decades ago, produces shoddy sperm. (The other male died in the fight.) It is also very old, likely over a century, Gerald Kuchling, an Austrian turtle biologist who has overseen the Chinese breeding program since the early two-thousands, said.

Artificial insemination has been attempted three times on the Suzhou pair, but no eggs have hatched. If conservationists can change that, population numbers could rebound rapidly. “It only takes a couple of years of breeding success and there could be hundreds of individuals,” McCormack told me. But even hatchlings present new problems: when the population comes from the same family, inbreeding becomes an issue. Moreover, all of the structural problems that decimated the turtles in the first place persist. Dams continue to be built along southeast Asian rivers, wetlands are being developed, and demand for turtle meat still tempts hunters.

Perhaps Dong Mo can serve as an example of local conservation success. Through negotiation with land managers, the A.T.P. was able to create a no-fishing zone within two portions of the lake, giving the turtle its own hunting grounds. The organization has also educated local communities about the turtle. Conservationists give lectures at schools, hang up flyers in villages, and even host the Turtle Football Cup, an annual soccer tournament on the banks of Dong Mo. Winners get a large poster of the turtle.

Back at the lake, we pulled the boat up to a small isle whose banks were feathered with green ferns. Water lapped against the hull of the boat, making metallic thumps. Surrounding the isle, a ring of small white buoys marked a no-fishing zone. The conservationists lunched on rice and pork and sipped ginger-flavored Cokes. Suddenly, Thang pointed toward the lake. His finger shook. The turtle had arrived.

Fifty yards out, near one of the buoys, was a head. Ha passed me binoculars. The head, coconut-size and with giant flared nostrils, was painted in squiggles of lime and pine green. Its bottom jaw was yellow. Sparkling in the sunshine was the long, thin bulge of the turtle’s carapace. We watched it floating there for several minutes, passing the binoculars back and forth, keeping quiet. Then, as quickly as it had appeared, it was gone.