Since 1990, when officials first began searching in Germany for unexploded ordinances, more than 130 such weapons have been uncovered. After the war, Germany recovered, but did so unevenly.
West Germany enjoyed the support of the Allies from the United States and Western Europe. The country developed into a commercial and industrial force. East Germany lagged behind, however, until the two states were reunited following the fall of the Berlin Wall. Then, East Germany underwent massive redevelopment efforts to catch up to the West, all funded by German taxpayers. These development efforts can lead to the discovery of unwanted historic artifacts.
In some cases, bombs can be moved. In other instances, they have to be detonated because the ordinances are simply too deteriorated and fragile to risk moving them. In the case of the 550-pound bomb, the device was equipped with a chemical trigger instead of a mechanical one, making simple defusal much more risky.
As Rose Eveleth writes on Smithsonian.com, the 550-pound bomb isn't even the largest recent discovery. Last year, German officials discovered a 1.8-ton explosive dropped by the British Royal Air Force during World War II near Koblenz. In that case, however, bomb removal experts defused the device simply by wrenching off the fuse.