History venerates the builders of great bridges, dams, and towers. But rare are commemorative plaques for the un-builders—those charged with the equally heroic task of dismantling those grand structures, once they become dowdy, obsolete, or downright dangerous. Herewith, five case studies in the art of mega-destruction—starting with the old, seismically shaky eastern span of the San Francisco–Oakland Bay Bridge. Also: remodeling NASA’s rocket assembly building, scrapping the world’s longest aircraft carrier, recycling a supercomputer, and moving a river to remove a dam.

1. Last Exit for the Bay Bridge

The original eastern span of the San Francisco–Oakland Bay Bridge, photographed from the Oakland side on November 8, 2013. The new bridge can be seen at right. Spencer Lowell

Demolition began: November 2013 | Duration of project: 3 years

Built during the Great Depression, the eastern span of the San Francisco-Oakland Bay Bridge was a marvel of utilitarian efficiency. (Some of the construction cranes were even incorporated as part of the structure.) But now that its graceful replacement is operational, the old span has to be taken down—without dropping anything into the water. Expected cost: $240 million. —Eric Smillie

To figure out the best sequence for removing the high-tension pieces, engineers will use a 3-D model, based on structural analysis and historical records, that shows how the forces are distributed. Bryan Christie Design