The Susquehanna River heads southward from eastern Pennsylvania and empties into Maryland's Chesapeake Bay. As the bay passes Baltimore it begins to widen. At one point it separates Maryland's mainland from the peninsula by 22 miles. The center channel is deep enough to allow ocean-going vessels to pass and is part of the intracoastal waterway. The inlet from the Atlantic enters between Norfolk and the tip of the Delmarva Peninsula. To accommodate large ocean going freighters, designers and engineers devised an interesting combination of bridges and tunnels to span the Chesapeake. The northbound section of U.S. Route 13 was opened in 1964, after 42 months of construction. The southbound project was completed in 1999. The 20 mile bridge-tunnel combo carried a price tag of just under half a billion dollars, and made the list of one of the seven engineering wonders of the modern world.