But later that night, his dad spotted the frog hopping around the ivy. They named it Lazarus, after a character in a Bible story who comes back to life.

Frogs and toads have a way of bouncing back. They have survived major extinctions, even the one that wiped out the dinosaurs. But Jennings, now 42, is worried. Lately, frog species have been disappearing from the earth. Foothill yellow-legged frogs, like Jennings' Lazarus, are gone from 45 percent of the areas in California where they once lived. In other parts of the U.S., people have been finding frogs that are deformed. Worldwide, many species that were common are becoming hard to find.

Jennings was among the first scientists to realize there was trouble in the amphibian populations. From 1988 to 1993, Jennings and fellow expert Mark Hayes studied the aquatic animal populations of the entire state of California. They had a very hard time finding red-legged frogs, yellow-legged frogs and Cascade frogs in areas where, just 20 years before, those species had been plentiful. "It was a real shock," Jennings says.

He and other scientists have successfully nominated two frog species to the state's endangered-species list. His most recent concern is the mountain yellow-legged frogs. "As far as I can tell, there are under 100 adults left," he says. "It used to be the most common frog you'd run into in the mountains of Southern California."

Scientists in many countries are investigating the frog mystery. They are examining pollution, overexposure to sun and other possible dangers, but so far they don't know just why the frogs are disappearing.

At first, some biologists doubted Jennings' findings. He told them to go out frog hunting themselves. "Once you go out and look, there's no question," says Jennings. "There really is a problem."