The single loud boom some Niagara-on-the-Lake residents heard late Tuesday night was the sound of police explosives experts detonating a Second World War-era artillery shell that was found in Lake Ontario.

A retired police officer who was exploring the lake bottom with a bathyscope not far from the old defence department rifle range, discovered the shell in about 50 feet of water.

Niagara Regional Police noted the area west of the old range historically was used for Second World War military training.

It was unclear whether the shell was still “live,” so after bringing it ashore, officers blew it up just before 9:30 p.m. as a precaution.

Police cautioned anyone finding objects resembling explosives or ammunition not to touch them and to call authorities immediately.