LOS ANGELES – When the research vessel Sally Ride set sail for Santa Catalina Island to map an underwater graveyard of DDT waste barrels, its crew had high hopes of documenting for the first time just how many corroded containers littered the seafloor.

But as the scientists on deck began interpreting sonar images gathered by two deep-sea robots, they were quickly overwhelmed.

The dumpsite it turned out, was much, much bigger than expected. After spending two weeks surveying a swath of seafloor larger than the city of San Francisco, the scientists could find no end. They could've kept going in any direction, they said, and uncovered even more.

"I was pretty shocked that it just kept extending as far as it did," said Eric Terrill of San Diego's Scripps Institution of Oceanography, who led the mission of 31 scientists and crew members. "We couldn't keep up with the flow of data coming in."

Terrill shared these findings Monday in a U.S. congressional briefing led by Sen. Dianne Feinstein, D-Calif., who has been pushing for action since the Los Angeles Times reported last fall that the nation's largest DDT manufacturer once dumped its waste into the deep ocean. As many as half a million barrels could still be underwater today, according to old records and a recent UC Santa Barbara study that provided the first photos of this pollution bubbling 3,000 feet under the sea.

"This mission confirms my worst fear: that possibly hundreds of thousands of barrels and DDT-laced sediment were dumped just 12 miles off our coast," said Feinstein, who said she plans to ask the U.S. Justice Department to look into companies that may have illegally dumped and whether they can be held accountable.

For Terrill, trying to count the barrels was like the episode of "I Love Lucy" in which Lucy gets a job in a chocolate factory and struggles to keep up with a speeding conveyor belt. But as his team pored through gigabytes and gigabytes of sonar data, they were finally able to identify at least 27,000 barrel-sized anomalies — and more than 100,000 total debris objects on the seafloor — with the help of computer analysis.

The actual number of barrels could be even higher, he noted, because barrels that were half-buried by sediment, for example, may have been overlooked by the computer.

The two-week expedition, made possible by a unique partnership between Scripps and the National Oceanic and Atmospheric Administration's Office of Marine and Aviation Operations, had deployed two high-tech robots that alternated scoping more than 36,000 acres of seafloor with high-resolution sonar: As one robot worked underwater, covering about 140-football fields per hour, the other recharged, offloaded its data and got recalibrated by scientists on deck.