It was August 2019 and Costales, a conservation biologist at University of New Brunswick, had traveled with colleagues to a private reserve in search of rainfrogs—small, brown amphibians that look like fallen leaves. As the cool of the night set in, the team had already found nearly a dozen rainfrog specimens, a good haul by any standards.

Then one of the scientists noticed a bright fleck of green on a low-hanging leaf, and everyone crouched down in awe.

“There it was,” says Costales, “the legendary Atelopus mindoensis!”

Before that night, A. mindoensis—commonly known as the Mindo harlequin toad—hadn’t been seen alive in 30 years. Most believed the species to be extinct, a victim of the fungal disease called chytrid.

Over the past