After nearly disappearing for good, Sierra Nevada yellow-legged frogs are once again hopping happily around California’s alpine lakes.

Scientists are celebrating the comeback of the amphibians (Rana sierrae) in Yosemite National Park. Though they’re still endangered, Sierra Nevada yellow-legged frogs have made a “remarkably successful” recovery from the deadly amphibian chytrid fungus, researchers report this month in the journal Nature Communications.

“The lakes are alive again, completely transformed,” says study co-author Roland Knapp, a biologist at the University of California, Santa Barbara, to USA Today’s Elizabeth Weise. “You literally can look down the shoreline and see 50 frogs on one side and 50 on the other and in the water you see 100 to 1,000 tadpoles. It’s a completely different lake.”

Sierra Nevada yellow-legged frogs are small creatures measuring 1.5 to 3.75 inches long, according to the U.S. Fish and Wildlife Service. They live high in California’s Sierra Nevada mountain range, at elevations between 4,500 and 12,000 feet above sea level. The frogs inhabit marshes, ponds, lakes and streams, where they feast on bugs and other amphibians. They also serve as a source of food for birds, snakes, coyotes and bears.

Sierra Nevada yellow-legged frogs tend to have yellowish-orange bellies and dark, splotchy backs, but their coloring can vary widely—from greenish-brown to gray to red. They don’t have vocal sacks, so instead the frogs grind their teeth together underwater when trying to attract mates in the spring.

The frogs were once abundant throughout the Sierra Nevada. But, after the arrival of European settlers in the mid-19th century during the California gold rush, their numbers began to dwindle.

In addition to gold, miners also discovered more than 1,500 alpine lakes in California. The lakes were beautiful, but they were lacking in fish—so the miners began stocking them. The introduction of non-native species—including rainbow trout, grayling and Atlantic salmon—decimated the Sierra Nevada yellow-legged frogs.

Stocking ended in the 1990s, but even without help from humans, the non-native fish continued to reproduce and thrive. Then, in the early 2000s, the few surviving frogs in the Sierra Nevada faced yet another threat: the amphibian chytrid fungus.

The highly contagious fungus (Batrachochytrium dendrobatidis) causes chytridiomycosis, an infectious skin disease that has caused mass die-offs and extinctions among amphibians around the world. In 2014, with their populations crashing, Sierra Nevada yellow-legged frogs were added to the endangered species list.

But then scientists noticed something peculiar: In some places, Sierra Nevada yellow-legged frog numbers were increasing. It appeared that at least some of the small creatures—particularly those living in lakes without any non-native fish—had developed a resistance to the fungus.

“The frogs that survive better have certain variations in their genomes,” says Erica Bree Rosenblum, a biologist at the University of California, Berkeley, to ScienceNews’ Martin J. Kernan. “Since they’re the ones surviving, they’re passing their genes down, and over time the whole population is changing toward having these more favorable genetic mutations.”

Researchers decided to implement an ambitious plan to save the species. Starting in 2006, they began gathering up the fungus-resistant survivors and re-introducing them to other alpine lakes without fish.

Now, nearly two decades later, scientists say their plan worked. These Sierra Nevada yellow-legged frog populations are now mostly self-sustaining and have “a low probability of extinction over 50 years,” they write in the paper. They hope the successful reintroduction of Sierra Nevada yellow-legged frogs might serve as a source of inspiration for scientists working to save other species battling new diseases.

“These frogs have somehow figured out how to exist, even thrive in the face of this pathogen,” Knapp tells the San Francisco Chronicle’s Kurtis Alexander. “When I saw these frog populations recovering on their own, that was the first time in 15 years working on this species that I felt a glimmer of hope.”