Norfolk is an independent city in the Commonwealth of Virginia in the United States. With a population of 234,403 as of the 2000 census, it is Virginia's second-largest city behind its eastern neighbor, Virginia Beach. Norfolk is located at the core of the Hampton Roads metropolitan area, named for the large natural harbor of the same name located at the mouth of Chesapeake Bay. It is one of nine cities and seven counties that constitute the Hampton Roads metro area, officially known as the Virginia Beach-Norfolk-Newport News, VA-NC MSA. The city is bordered to the west by the Elizabeth River and to the north by the Chesapeake Bay. It also shares land borders with the independent cities of Chesapeake to its south and Virginia Beach to its east. One of the oldest of the Seven Cities of Hampton Roads, Norfolk is considered to be the historic, urban, financial, and cultural center of the region. The city has a long history as a strategic military and transportation point. Norfolk Naval Base is the world's largest such base, and the world's largest military alliance, the North Atlantic Treaty Organization has its defense headquarters here. The city also has the corporate headquarters of Norfolk Southern Railway, one of North America's principal Class I railroads, and Maersk Line, Limited, who manages the world's largest fleet of US-flag vessels. As the city is bordered by multiple bodies of water, Norfolk has many miles of riverfront and bayfront property. It is linked to its neighbors by an extensive network of Interstate highways, bridges, tunnels, and bridge-tunnel complexes.