Eleven munitions, ranging in size from 500 to 4,000 pounds, were found at the construction site, less than a half-mile from the tunnel. The site was once used by the Navy and contractors to assemble and disassemble ships.
The water entrance to the Inner Harbor had also been closed, forcing the diversion of several cargo ships.
The 13-mile tunnel carries Interstate 895 under the harbor, near Fort McHenry. Another, newer, tunnel, the Fort McHenry Tunnel, carries much of the Interstate 95 north-south through traffic.
Military investigators are trying to determine exactly where the ordnance came from and whether there are any more munitions at the site, said Cpl. Greg Prioleau, a spokesman for the Maryland Transportation Authority.
The closing was ordered by Gov. Robert Ehrlich on the recommendations of ordnance disposal teams from the U.S. Army, Baltimore City, the FBI and the Maryland State Fire Marshal. Adjacent portions of I-895 also were closed.
Some businesses in the part of Baltimore, including a Toyota import auto storage facility, were expected to be closed Thursday. Some ships were diverted from the harbor.
The first bomb was discovered Wednesday morning when a backhoe operator dug up what he at first thought was a pipe. When he went to take a closer look at the unusual object, he realized it was a bomb.
There are no residences near the site, so there were no homes evacuated.
The Maryland Port Authority had purchased the 10-acre waterfront property four years ago, viewing it as a potentially "hot property" because of its location. The polluted ground was being covered with three feet of dirt and, eventually, asphalt.