The bridge collapses.
100 years after people: The Leonard P. Zakim Bunker Hill Memorial Bridge in Boston has been weakened from 100 years of corrosion caused by rain water and salt deposited from bird droppings. Finally its cables fail and the bridge collapses. However the towers which held the roadway still stand for now.
120 years after people: After the roadway collasped 20 years ago the towers are on the verge of collapse as nature reclaims Boston. Finally the huge towers fail at the base the memorial is wiped for memory forever.