After years of scientific and legal wrangling, the U.S. EPA began an experiment yesterday off the Palos Verdes Peninsula in California to test whether DDT-contaminated sediments on the ocean floor can be sealed with a layer of mud to protect marine life and people from the pesticide. If it deems the pilot project on 135 acres a success, the agency hopes to begin a big effort in 2002 to cover three to four square miles of sea floor off the peninsula in a similar manner. But some marine experts say that the benefits of capping a pollution site in such deep water are not worth the cost and that the EPA solution could make the problem worse by stirring up the pollution. The EPA sued the Montrose Corp. and five other companies 10 years ago, seeking $170 million damages for the DDT pollution; the case goes to court in October.