Water leak inside Tunnel: Gridlock ensued when water began leaking into the northbound lanes of the tunnel, at one of its deepest points, just north of South Station at about 1:45 p.m. Wednesday. The tunnel carries Interstate 93 through the city. The water resulted in two lanes of the tunnel being closed to traffic underneath downtown Boston, shortly before the evening rush hour was to begin. Sibley explained that an eight-inch pocket of clay mixture in the tunnel walls was likely to blame for the leak. Water found its way into the pocket, and the high pressure at the tunnel's extreme depth -- 110 feet below the earth -- likely resulted in the leak, he said. Sibley said the leak, which occured in a portion of the wall which has been in place for 10 years, posed absolutely no structural threat to the tunnel itself.