Articles about the unsolved mystery of the "Berkeley Boom" claim that the loud noises that can be heard throughout Berkeley, California, and in neighbouring cities, might be caused by fireworks, cops, oil companies, nuclear testing, car engines, aerial bombs, flash grenades, and other theories.
The explosion-like-sound has been heard throughout the East Bay at irregular times and on irregular dates, triggering car alarms, freaking out pets, and disturbing the local population - and authorities claim they have no idea what causes it.
It seems strange to me that the origin of a noise - especially a threatening and scary one - would be hard to track down with today's technology, and would be something that authorities haven't figured out yet. And it seems strange too that all I could find online are theories.
Is it possible that the origin of a loud explosion is "unknown", even when it happens repeatedly over a long period of time?