In the 24 days that Humphrey the humpback whale explored the inland waters off San Francisco Bay, swimming as far as 70 miles in from the ocean, marine biologists got one of their longest looks ever at a single whale.

"I think we probably tripled the amount of literature and scientific knowledge [about humpbacks] in the past three weeks, just being able to observe the animal as we have," said Bernard Krause, a bioacoustics expert who prepared the tape-recorded sounds that enticed the 40-ton mammal back to the ocean.

At first, scientists tried to scare Humphrey seaward with tape recordings of killer whale sounds. Killer whales are believed to be a natural enemy of humpbacks, but Humphrey was unfazed.

Then they tried a method Japanese fishermen use to drive dolphins out of their tuna nets -- banging pipes underwater. It worked a little, but not enough to keep the whale heading the right way.

But playing taped sounds of humpbacks feeding did the trick.

Humphrey swam directly for the underwater speaker, trailed by a boat leading the way to the sea.

"It seemed like he wanted to get right inside that speaker," Krause said.

"He was eating the barnacles off our boat. That's how close he was."