In A.D. 9, German tribes entrapped and annihilated a Roman army of 15,000 men somewhere in northwestern Germany. The battle changed the history of Europe for the next 2,000 years. Known to history as the Battle of Teutoburg Forest (or Teutoburger Wald), its location was long lost to historians. Dozens of possible locations were suggested, but no one knew for sure.
In 1987, a British army officer serving in the area of Osnabrück did some research and developed a theory that the battle was fought nearby, at a hill named Kalkriese. He began scouring the area with a metal detector, and found numerous Roman coins, sandal nails, bits of armor, and other artifacts that confirmed his hunch. In the years since, archaeologists have been busy excavating the area, finding all manner of pieces — including human bones — that have definitively established this as the battle site.
Fergus M. Bordewich wrote an article in the Smithsonian Magazine a couple of years ago describing the battle and its aftermath, and the story of its rediscovery. Fascinating reading.
UPDATE (1/5/08): Jona Lendering has written a much more detailed description of the battle, including the history before and after, an analysis of the ancient literary sources of information, and the archaeological evidence. A must read for students of this battle.