Eleven hundred kilometers off the coast of Mexico, in the middle of the Pacific Ocean, sits a tiny hunk of rock and sand known as Clarion Island. No one lives on Clarion. Other than a small contingent of sailors from the Mexican Navy, who come and go every two weeks, the only people who visit Clarion are occasional groups of biologists and students who make the long journey to study the seabirds that nest there as well as the island's unique wildlife. The island is the only known home of the Clarion burrowing owl (Athene cunicularia rostrata), the critically endangered Clarion Island whipsnake (Masticophis anthonyi), the Clarion Island tree lizard (Urosaurus clarionensis) and several other native species.

Biologists have long thought that they had described all of the island's species. But did they miss something? Back in 1936 a naturalist named William Beebe visited the island and wrote about another vertebrate that no one else had seen since. He called it the Clarion Island nightsnake. In a paper published 10 years later he identified it as a subspecies of similar snakes from the genus Hypsiglena that are found up and down the west coast of North America.

But at least one later researcher discounted this discovery. B. H. Brattstrom visited the island several times in the 1950s and never saw the nightsnake. He decided that Beebe must have made a mistake cataloguing the snake. Beebe's expedition had traveled thousands of kilometers and collected hundreds of specimens. Brattstrom argued that Beebe had collected the specimen elsewhere, and he struck the nightsnake from the record. "It's conceivable," acknowledges Daniel Mulcahy, a researcher with the Smithsonian’s National Museum of Natural History (NMNH). "But on the other hand Beebe was a very well-known naturalist. He took very good notes. For him to make an error like that, you wouldn't expect it."

Mulcahy first heard about Beebe's nightsnake a decade ago while working as an undergrad at Berkeley, where he studied the Hypsiglena genus. "I was really amazed how many subspecies were described," he says, "particularly from all of the different islands around Baja and the mainland." At the time he wasn't aware that Brattstrom had removed Beebe's snake from the record, so, he says, "I went on thinking there was a snake on this island that I'd never be able to visit."

Flash forward to 2012, when Mulcahy started looking at images of Clarion Island on Google Earth. He noticed several photos of the island's birds and plants and lizards, but no nightsnake. Mulcahy started corresponding with biologists, including Juan Martinez-Gómez, who has been working on the island for 25 years. Many people had seen the larger, diurnal whipsnake, but no one had ever reported seeing anything like Beebe's nightsnake.

Mulcahy then started some real detective work. He borrowed Beebe's original specimen from the American Museum of Natural History in New York. He asked for a list of everything that Beebe collected on the expedition, so he could retrace his steps. And then he tracked down a copy of a book that Beebe published in 1938 recounting his 1934 expedition. There, in the final pages, came the final piece of the puzzle. Beebe wrote about visiting a beach watching sea turtles build nests when he came across a 45-centimeter snake moving against an outcropping of black lava rock: "I saw a small, dark brown snake," he wrote in the book, Zaca Venture. "It seemed to be unlike the one I had found in the daylight"—the much larger whipsnake—"having lines of black spots on the body."

"I jumped up and said, 'That's it!,'" Mulcahy remembers. He knew that Beebe really had collected the nightsnake on Clarion Island. Martinez-Gómez encouraged Mulcahy to travel to Mexico to see if they could find the snake once again. "It was a big risk," Mulcahy admits. "I'm a technician at a molecular lab. They don't pay me to go out and chase things. I had to get funding elsewhere." The journey—which required approval from the Mexican Navy—would also take more than a month, because Mulcahy would be joining a team that would spend 15 days on nearby Socorro Island before making the 16-hour ocean journey to Clarion for another 15-day stay.

He decided to take the risk. Last year, on May 19, after thinking about the Clarion nightsnake for 10 years, he finally landed on Clarion Island, near the same beach that Beebe wrote about in Zaca Venture. His heart sank when he arrived. The first thing they noticed was that all of the vegetation was burnt and scorched. "All of the bushes were shriveled and twisted like twigs," he says. "It turned out the military had a controlled burn that got out of control. It scorched the area."

But that afternoon they started looking at the area, using Beebe's photos to retrace his steps from 79 years earlier. "Along the eastern edge of the beach there's a small knoll of black rock. We thought, that's good habitat. It's not burned." They decided to explore it that night. "We were hoping for a lot, but not expecting much that first night," Mulcahy acknowledges. "The four of us were talking and finding lizards and spiders and things." Then one of the students on the expedition, Juan Cervantes, shouted two magical words: "Una culebra!"—a snake. "We all ran over," Mulcahy says. Cervantes pointed into a hole and said the snake had gone inside. "It was a small hole. I removed a softball-sized rock and underneath—there it was. I thought, 'Oh my god, we did it.' We quickly pulled it out. It was amazing. We were jumping up and doing high fives."

They found one more snake that night, and a total of 11 over their 15 days on the island. They had proved that the Clarion Island nightsnake existed. "It was just a major relief," Mulcahy says. "Investing all of that time and effort to get out there with the chance that we were out there at the wrong time or we couldn't find them..." His voice trails off.

After returning to the U.S., Mulcahy conducted genetic tests on the samples they collected. They revealed that the Clarion Island nightsnake was not a subspecies but was actually a new species, which they dubbed Hypsiglena unaocularus. The species was described this month in PLoS One.

Mulcahy believes the snake should now be considered an endangered species. "These island ecosystems are fragile and easily disturbed," he says. Clarion in particular faces two nasty invasive species: feral pigs and rabbits, which were probably brought to the island decades ago as food but have since gotten out of control. Nearby Socorro Island also has a large population of feral cats, which have wiped out populations of several bird species that did not evolve with the threat of predators. The invaders have already changed Clarion. When Beebe wrote about it, he described it as an island filled with impenetrable cacti. His expedition needed machetes to make their way through the cactus fields. Those plants are now almost all gone, probably eaten by the pigs and rabbits as a source of water—Clarion has no freshwater of its own. "We found less than a dozen prickly pear cactus plants on the island in the 15 days we were there," Mulcahy says. "They've been almost completely obliterated." The long-term effects of this extirpation remain to be seen.

There's still a lot left to learn about the Clarion Island nightsnake now that we know it exists. "We don't have any direct evidence of their diets or their ecosystems," Mulcahy says. We also don't know how many remain on the island. But that information may come. For him, he says, "it has been nearly a decade of me thinking, well, maybe I'll make it out to Clarion. Now I've put a major flag in the ground to move forward." And a mystery nearly 80 years in the making has been solved.

Photo courtesy of Daniel Mulcahy