The humpback whales finally returned in the ocean. The marine biologists found no trace of the mother and calf whales that had been stranded in a stretch of California's Sacramento Delta. They hope that the pair of marine mammals had returned overnight to their natural Pacific Ocean habitat.

Last time when the two whales, nicknamed Delta and Dawn were seen they were just 10 kilometers from open water. Personnel on boats and helicopters found no trace of them Wednesday morning prompting speculation that they had slipped under the Golden Gate Bridge and out into the ocean overnight.

"They arrived here quietly, and if they've made their way outside the Golden Gate, they've gone out quietly," said Bernadette Fees, a California Fish and Game spokeswoman.

"They could be anywhere," said Carol Singleton, spokeswoman for the state Office of Emergency Services. "The hope is that they're home and they're safe."