WASHINGTON (Reuters) - Washington is starting to dig deep in a $2.6 billion underground solution aimed at helping clean up the polluted Potomac River and the ailing Chesapeake Bay, the biggest U.S. estuary. In the U.S. capital's biggest public works project in more than 40 years, work started this fall to cut about 16 miles of tunnels to keep overflow sewage and stormwater from running into the Potomac. The project, designed to be finished in 2025, is seen by environmentalists as part of resolving the next great water pollution challenge facing the United States -- keeping fouled runoff out of...