Crossing Lake Washington in a car takes only a few minutes, but those minutes carry the weight of extraordinary human achievement beneath your tires. 77 Pontoons Keeping It All AfloatThe secret to this highway floating on water comes down to 77 watertight precast concrete pontoons sitting just below the surface of Lake Washington. They were precast in Aberdeen, Washington, then towed approximately 400 miles through open water before being carefully positioned and assembled on Lake Washington. Keeping a 7,708-foot bridge from drifting requires a serious anchoring system, and the SR 520 bridge delivers with 58 large anchors embedded into the floor of Lake Washington. It can whip up serious waves during Pacific Northwest storms, and the SR 520 bridge was engineered specifically to survive them.