An explosion goes off under a runway. The article says "it's a bomb, it was dropped on the runway 80 years ago, buried with dirt, paved over, then it went off".

Cool. Now, how did you figure that out? Did someone analyze the shrapnel and conclusively date it to allied bombing campaigns in WWII, or is that what a source close to the events said, or did the reporter just... look at the picture and shoot from the hip? Any of these is an acceptable answer, so long as readers know which one it is.