The Millennium Bridge crosses the Thames without a lot of fuss, but many contemporary footbridges are called upon to be instant landmarks, much like iconic buildings. No one has built more landmark bridges than Spanish architect-engineer Santiago Calatrava. This footbridge crosses over the Rio de la Plata in the port of Buenos Aires. The 525-foot walkway is supported by stay cables strung from a dramatic steel pylon like harp strings. The swing bridge rotates 90 degrees, in order to permit river traffic to pass. As in many of Calatrava's structures, this sculptural solution is not as straightforward as it first appears. Since the mast is leaning over the deck, the open bridge is inherently unbalanced. To support the weight of the bridge, the portion of the hollow pylon beyond the swivel point is filled with concrete and acts as a hidden counterweight.