MIT Building 20
At the Massachusetts Institute of Technology (MIT), buildings are traditionally assigned numbers based on a grid system rather than names (although over the years, as at most universities, some buildings have acquired the names of donors). In 1943, during World War II, the famous MIT Radiation Laboratory (Rad Lab) was established by the U.S. military to conduct America's radar research. Building 20 was built using Quonset hut construction to house the new program. The feeling was that the building need last only as long as the project, which would be for the duration of the War. Indeed the Rad lab closed in 1945, but Building 20 was not torn down. It continued to be used for various MIT activities that needed special facilities or did not fit into standard departmental structures. Many important innovations came out of Building 20, to the point where it came to be nicknamed "The Magical Incubator."
Finally, in 1998, the "temporary" structure was demolished to make way for a new complex of buildings, 55 years after it was built!
MIT maintains a website documenting the history of Building 20 at http://www.eecs.mit.edu/building/20/