We were shook at 12:20 to the sound of an explosion. We couldn't locate the direction of the source due to the tall buildings that surround our apartment. Within five minutes, sirens directed up to the Dane County Parking ramp, located two blocks west of our place. We followed them to the scene, along with some others wondering what happened. All concurred that there was an explosion, some relating stories of pictures shaking on walls. The emergency crews were not in a hurry, but diligent in their duties. The only rumor we heard as to the source was that a car exploded and a man staggered out of the ramp claiming that it was his.
Approximately one month ago, a pipe bomb was discovered at the same structure by a worker. It was diffused and police had no leads.
As we started to leave, so did the plethora of rescue vehicles. I took the above shot as we started to walk away.
Update 12/25: Turns out it was a bomb. The previous bomb scares were not bombs, just resembled bombs.