It had been more than 140 years since a Rio Grande siren—a slippery, two-legged, foot-long salamander protected by the state of Texas—had been found near Eagle Pass, a town on the United States-Mexico border. But in 2019, biologist Krista Ruppert, now a PhD student at Mississippi State University, realized she didn’t need a siren in hand to prove they were still there.

She just needed a pitcher of muddy water to filter.

At Eagle Pass, Ruppert found enough environmental DNA—trace genetic material left behind as organisms crawl, swim, or flap their way through life—to establish that the elusive amphibians still live in the area, at the far western edge of their known range.

In the