HAGERSTOWN, Md. (AP) — The National Park Service says it's temporarily closing one end of the Paw Paw Tunnel in the Chesapeake and Ohio National Historical Park for safety reasons.

The agency said in a statement Wednesday that the northern, or downstream, portal of the half-mile tunnel will be closed for four to five months, starting in mid-February. The closure will enable workers to remove loose rock and unstable surface materials.

The park service says visitors still will be able to enter the tunnel through its upstream portal and walk to the northern portal.

The tunnel, about 150 miles upstream from the District of Columbia, is the largest structure along the canal. The canal parallels the Potomac River for nearly 185 miles from Washington to Cumberland.