When a major highway bridge in Minneapolis, Minnesota, fell on Aug. 1, 2007, the nation was shocked. Speculation about the cause of the collapse was widespread in the following days. Was it metal fatigue? Corroded steel roller bearings? A terrorist attack? Fifteen months after the disaster, the National Transportation Safety Board released an extensive investigation report. The proximate cause (gusset plate failure) surprised engineers and a public familiar with other bridge failure modes. The riveted metal plates connecting structural beams were assumed to be stronger than the beams themselves, but were in fact too thin to withstand years of ever-increasing loads. NASA may not build many bridges, but the I-35 story teaches us that hidden hazards can lurk within aging structures.