Authorities in the Philippines were trying to drive back to sea a pod of about 200 dolphins that had swarmed to shallow waters in Manila Bay on Tuesday morning.
"This is the first time as far as I can remember that so many dolphins are inside Manila Bay and acting so erratically," said Malcolm Sarmiento, director of the Philippines' Bureau of Fisheries and Aquatic Resources.
The dolphins were spotted about a mile offshore near the townships of Pilar and Orion in Bataan province, he said.
Volunteers and rescue workers kept them from beaching and were using about 20 boats to push them back toward deeper waters.
"We prevented a massive stranding. We're now trying to find out what exactly is causing this strange behavior," Sarmiento said.
The dolphins may have headed toward the bay for two reasons, he said.
"A sea quake perhaps," he said. "If they were underwater, the water pressure would injure their eardrums."
The pod might also have been following a sick or wounded leader. If so, rescue workers will have to find the weakened dolphin and isolate it from the pod.
"Hopefully, the healthy can go back to sea if the leader's out of sight," Sarmiento said.