In the 19th century, lightships, which are essentially floating lighthouses, were stationed in coastal waters where lighthouses could not be built. A lightship displayed one or more lights from a mast or masts as an aid to navigation. At one time or another, more than 120 lightships dotted the coastal waters of the United States. By 1985 all U.S. lightships had been replaced by buoys that are equipped with an automated beacon and fog signal. These massive buoys, called Large Navigational Buoys, measure 12 m (40 ft) in diameter and are among the largest of a variety of a variety of navigational aids known as lesser beacons. Lesser beacons include river lights, fog signals, and numerous other smaller navigational aids used to mark channels in rivers and harbors. Today hundreds of thousands of lesser beacons are in use throughout the world.