Nick Kamp, who works at a water treatment plant in Paso Robles, Calif., was near a solid-waste pond when he spotted something emerging from the murky water. He quickly snapped a few photos of what turned out to be a capybara, the world's largest rodent, before it slipped back into the water and swam away.

Capybaras resemble huge guinea pigs and can grow to be as large as sheep, sometimes reaching more than 1.3 meters in length. They eat grass, freshwater plants and—to aide digestion—their own feces. The semiaquatic mammals are native to South American marshlands and swamps, but this sighting in central California was not the first.

Three years ago, a capybara sighting at a nearby golf course was dismissed as a probable beaver misidentification, but the hulking rodent later showed up chasing a farm dog, whose owner tried unsuccessfully to shoot the wild animal. California Department of Fish and Game official Todd Tognazzini told local TV news reporters that he suspects people are seeing the same capybara in each incident, and that it's most likely an escaped exotic pet.

This sighting was the first caught on film.

Lauren F. Friedman