After a strong and sudden storm on June 29 cut power to 443,000 customers in Washington, D.C., and its Maryland suburbs, utility Pepco said it is considering burying more power lines. The company has no cost estimate yet to place its system underground, but experts have estimated it could be as high as $5 billion since lines must be buried in 3-ft-deep trenches, said a spokesman. "Pepco will analyze the benefits and costs of more extensive selective undergrounding and even complete undergrounding of the overhead electric system," he said. The utility told the Maryland Public Service Commission in a July 13 letter that even aggressive tree trimming cannot always resolve reliability issues on some lines, particularly in catastrophic storms. Pepco, which already has replaced 500 miles of aging underground lines, will file its analysis with regulators in six months.