It's been a mystery for months on the Pacific coast of Peru, where the local government says it has found 900 dolphin carcasses and something like 4,500 pelicans. It's been bad enough that the country's health ministry ordered 1,500 miles of beaches closed.
And while it may all seem very far away from the United States, scientists from around the world have been watching. People in the area say the government has been slow to take up the bodies, and slower to solve the puzzle.
Which leaves a lot of room for finger-pointing. Every group has its own explanation for the animal deaths:
The government has said the dolphins died of disease: "The most probable hypothesis is the possibility of an infection with a virus," Deputy Environment Minister Gabriel Quijandria told The Associated Press.
Environmental groups say dolphins' inner ears were literally fractured by seismic blasts set off by U.S. oil prospectors: "The ears were soaked in blood. That's not normal when you examine a bone," said Dr. Carlos Yaipen-Llanos, president of the activist group Orca.
Other scientists wondered about agricultural runoff or heavy metals from mining near rivers, though the Peruvian Sea Institute said it did not find unusual chemical concentrations in animals it autopsied.
Actually, more than one argument may be right; some biologists say the dolphins and the birds probably died for different reasons. But the theory that's been gaining the most traction in recent days involves the global climate. And if it's accurate, then Peru seems a little less far away.
Remember the El Nino phenomenon? An El Nino is a giant patch of warm water, thousands of miles long, that periodically appears along the equator in the Pacific. It alternates with a La Nina -- a patch of unusually cold water. They are large enough to alter weather patterns around the world; during El Nino periods, for instance, jet streams, picking up energy from the steamy Pacific, can tear apart hurricanes in the Atlantic -- good news if you live on the coast between Texas and the Carolinas.
The Pacific has just moved from a La Nina period -- cold water on the equator -- to a relatively neutral phase, but it did it unusually quickly, and that got meteorologists thinking.
"It's pretty warm out there," said Jim Andrews, an operational meteorologist at AccuWeather, the Pennsylvania-based private weather forecasting service. "I'm not a biologist, so I can't draw a straight line from the ocean temperature to the birds' deaths. But I wanted to offer the possibility that there's a connection."
Off the Peruvian coast, where it is now autumn, ocean temperatures have been reported to be 10 degrees Fahrenheit above normal for this time of year. Sea animals can survive that -- but some have trouble adjusting.
Several biologists have suggested that because of the temperature change, anchovetas (a type of anchovy) have been moving into deeper water to stay cool. That's fine for them -- but they're a dietary staple of pelicans, which can no longer dive down far enough to reach them for food. Carlos Bocanegra, a biologist at the National University of Trujillo, said he did analyses of 10 young dying pelicans, and found their digestive tracts were either empty or contained fish the pelicans don't normally eat.
The theory holds water because pelican die-offs have happened before. AccuWeather's Andrews said that in 1997, just as an El Nino period began, there was a major die-off.
"It's not rare that this occurs," said Patricia Majluf, a former deputy fisheries minister. "It looks ugly because this has occurred at the same time and place [as the dolphin deaths]."
The Climate Prediction Center of the National Weather Service in the U.S. says that by the end of summer in the northern hemisphere, El Nino should return full-force to the Pacific. No saying yet what it could mean for the dolphins and the birds of the South American coast.
The Associated Press contributed to this report.