[Mac Cat]

I read a really interesting article on the same topic, while I was visiting Berlin this summer.

Many of the biggest bombs we dropped near the end of the war didn't go off, because they hit in swampy, wet areas near rivers (as in Berlin), where the bomb traveled under the mud and slid down, horizontal and then started back up when they finally stopped moving (a U-shaped trajectory in the soft earth). Air crews reported many of the locations, but obviously not all of them. After the war, people built homes right on top of the bombs, while others were found deep under existing buildings.

The detonators were time delayed, using a corrosive chemical in a small glass jar, which apparently didn't break open, particularly when the bomb was found in an upright position. Usually, they can't be safely removed, so the German engineers excavate around them, then cover them with heavy blasting mats, and detonate them in place.

Every few months there's another story about finding unexploded stuff in France and Germany, but they still have large areas that are mined and off-limits, even today.

I didn't read the Smithsonian article, but I'll look it up.