Beijing - First, the water level in a pond inexplicably plunged. Then, thousands of toads appeared on streets in a nearby province. Finally, just hours before China's worst earthquake in three decades, animals at a local zoo began acting strangely.

As bodies are pulled from the wreckage of Monday's quake, Chinese online chat rooms and blogs are buzzing with a question: Why didn't these natural signs alert the government that a disaster was coming?

"If the seismological bureau were professional enough they could have predicted the earthquake ten days earlier, when several thousand cubic meters of water disappeared within an hour in Hubei, but the bureau there dismissed it," one commentator wrote.

In fact, seismologists say, it is nearly impossible to predict when and where an earthquake will strike.

Several countries, including China, have sought to use changes in nature - mostly animal behavior - as an early warning sign. But so far, no reliable way has been found to use animals to predict earthquakes, said Roger Musson, a seismologist with the British Geological Survey.

But that has not stopped a torrent of online discussion. Even the mainstream media has chimed in, with an article in Tuesday's China Daily newspaper questioning why the government did not predict the earthquake.

Online commentators say the first sign came about three weeks ago, when large amounts of water suddenly disappeared from a pond in Enshi city in Hubei province, around 350 miles east of the epicenter, according to media reports.

Then, three days before the earthquake, thousands of toads roamed the streets of Mianzhu, a hard-hit city where at least 2,000 people have been reported killed.

Mianzhu residents feared the toads were a sign of an approaching natural disaster, but a local forestry bureau official said it was normal, the Huaxi Metropolitan newspaper reported May 10, two days before the earthquake.

The day of the earthquake, zebras were banging their heads against a door at the zoo in Wuhan, more than 600 miles east of the epicenter, according to the Wuhan Evening Paper.

Elephants swung their trunks wildly, almost hitting a staff member. The 20 lions and tigers, which normally would be asleep at midday, were walking around. Five minutes before the quake hit, dozens of peacocks started screeching.

There are a few possible reasons for such behavior, said Musson, the seismologist. The most likely is that the movement of underground rocks before an earthquake generates an electrical signal that some animals can perceive. Another theory holds that other animals can sense weak shocks before an earthquake that are imperceptible to humans.

Zhang Xiaodong, a researcher at the China Seismological Bureau, said his agency has used natural activity to predict earthquakes 20 times in the past 20 years, but that still represents a small proportion of China's earthquakes.

"The problem now is this kind of relationship is still quite vague," he said.

In winter 1975, Chinese officials ordered the evacuation of the city of Haicheng in northeastern Liaoning province the day before a 7.3 magnitude earthquake, based on reports of unusual animal behavior and changes in ground water levels. Still, more than 2,000 people died. Strange environmental phenomena including changes in well water levels, were also reported a year later before a 7.6 magnitude earthquake in Tangshan in northeastern China that killed 240,000, Musson said.

A team of Chinese seismologists was sent to the region but didn't find any evidence to suggest an earthquake. As the seismologists were going home, they stopped for the night in Tangshan and were killed in the quake.