The Tower Bridge, which was built in 1894, is the only remaining bascule (a type of mechanical arch) bridge in London. The opening and closing of the valves on the sides of the tower raise or lower the bridge deck with great speed and ease. When lowered, the main span of the bridge becomes a flat floating surface about 120 feet above the waterway. The idea of a bridge with a floating section had been proposed before but never actually constructed. It was an innovation that made possible the construction of long, thin bridges without piers. As the name suggests, bascule bridges use counterweights to open and close their spans. When the bridge is up, the weight of the traffic forces a cable down through the center of the roadway. At the base of the tower, the cable pulls a large arm sideways, lifting the middle section off the ground. When the bridge is down, the weight of the counterweight again forces the cable down, this time through a drum at the base of the tower. The Tower Bridge is a world-famous landmark that attracts millions of visitors every year.