What caused the ‘boom’ heard in O’Fallon? This time, police have an answer.

Metro-east residents might have noticed a loud boom on Wednesday night.

And while some booms heard in the region remain a mystery, O’Fallon police have the answer for what caused this one.

At 7:45 p.m. Wednesday, Scott Air Force Base’s explosive ordnance disposal team conducted a controlled detonation of old military munition unearthed by Caseyville Township Sewer System employees digging for a sewer line.

The sewer system employees reported the object to police on Wednesday, and they contacted the military explosives team, the O’Fallon Police Department said in a news release Thursday.

The military team determined the object was a live 90mm projectile estimated to be approximately 80 years old.

In late December, residents as far apart as Red Bud and Edwardsville described hearing and feeling a boom with no explanation for what caused it. In that case, it wasn’t Scott Air Force Base or other known military operations, officials said. No cause has been reported.

This story was originally published February 6, 2025 at 11:36 AM.