PITTSBURGH (KDKA) – You never know what you can find in the Monongahela River — just ask Brianna Higby and Nick DiLiscia.

“It was definitely an awesome find,” DiLiscia says.

The two were magnet fishing over the weekend near the Homestead Grays Bridge along the Mon River.

“When we first found it, we didn’t know what it was because it was completely rusted and covered in sludge,” DiLiscia says.

“It took, like, three tries to get it,” Higby says. “It kept slipping off the magnet because it was so bottom-heavy, it would just slide right off.”

When they cleaned it off, the two got a better idea of what they had found: an artillery shell.

So, how long had it been in the Mon River and how did it get there?

Well, after talking to experts at the Heinz History Center, KDKA’s Brenda Waters was able to get some possible explanations.

Possibility number one: could it have come from the B-25 bomber that crashed into the Mon River back on Jan. 31, 1956?

That plane was never found and remains an unsolved mystery. Two of the six crew members died, their bodies recovered months later.

Possibility number two: could the shell have come from the old Hays Army Ammunition Plant that manufactured 105 mm shells from 1942 until 1970?

The plant made 250,000 of the shells per month.

After seeing the video, experts at the History Center believe it is an unloaded post-World War II American howitzer 105 mm shell.

It is unlikely to have come from that B-25 bomber because it doesn’t appear to have been on the floor of the Mon for more than 60 years.

Best guess: someone dumped the shell there in recent years.

Yet another mystery from the depths of the Mon River.