At 5:21 in the afternoon of 9/11, almost seven hours after the Twin Towers had come down, Building 7 of the World Trade Center also came down. The collapse of this building was from the beginning considered a mystery. 
The same should have been true, to be sure, of the collapse of the Twin Towers. But they had been hit by planes, which had ignited big fires in them, and many people assumed this combination of causes to be sufficient to explain why they came down.
But WTC 7 had not been hit by a plane, so it was apparently the first steel-framed high-rise building in the known universe to have collapsed because of fire alone. New York Times writer James Glanz quoted a structural engineer as saying: "[W]ithin the structural engineering community, [WTC 7] is considered to be much more important to understand [than the Twin Towers]," because engineers had no answer to the question, "why did 7 come down?"