The Evergreen Point Floating Bridge, also known as the 520 Bridge and officially the Governor Albert D. Rosellini Bridge, carries Washington State Route 520 across Lake Washington from Seattle to its eastern suburbs.
The 7,710-foot-long (2,350 m) floating span is the longest floating bridge in the world, as well as the world's widest measuring 116 feet (35 m) at its midpoint.
Beneath the surface of the water, 58 anchors secure the pontoons to the bottom of the lake, connected by three-inch-thick steel cables. As with the former bridge, a pontoon structure was deemed more practical than a fixed bridge due to the middle of Lake Washington being 200 feet deep with a further 200 feet of soft silt below it. Building the foundations for a suspension bridge, therefore, would have been a hugely costly and complicated endeavor.
On the water float the 77 concrete pontoons, the largest of which are 75 feet wide and 360 feet long.
The idea of building a bridge out of massive floating chunks of concrete may sound crazy, but each pontoon has a watertight compartment – remotely monitored to detect any leaks – and the weight of the water displaced by the pontoons is equal to the weight of the structure and all the traffic on it, allowing the bridge to float.
According to en.wikipedia & atlasobscura