A mysterious green cantaloupe-sized metal ball with an unknown firing device, found in 2005 by GIs and a freelance journalist in Mosul, Iraq, is confounding weapons experts trying to pinpoint its origin.

Dubbed "Mystery Weapon 2" by Iraq-based journalist Michael Yon, the orb was found under a barn among a weapons stockpile that was later destroyed by the 1st Battalion, 24th Infantry Regiment.

"If there's anything that, honest to God, could have nefarious overtones, it's that, because nobody was able to tell what it was," Yon said.

FOXNews.com in cooperation with Yon last week published a photograph of a weapon of unknown origin that was believed to be either some sort of rocket-propelled grenade launcher or a mockup. It, too, was found in the Mosul cache.

Click here for Michael Yon's description of "Mystery Weapon 2."

Think you know what Mystery Weapon 2 is? E-mail your guess to: mysteryweapon@gmail.com. We'll review your "expert research" and publish the best guesses... One clue: it's not a toy.

While "Mystery Weapon 1" looks as if it might be a homemade weapon, this greenish sphere is definitely the product of some sort of munitions manufacturer, Yon said.

"Now that thing, in particular, I thought 'Wow, that looks like either maybe some sort of cluster munition or maybe a chemical munition,'" Yon said. "And I asked a lot of different people, I've showed them that photo, and even the experts are like, 'You know, I don't know. I don't know what that thing is.'"

Yon recounts with amazement the harrowing moment when the soldiers pulled out this mystery weapon.

"When they pulled that out, I said something like 'Stop! Stop what you're doing, don't move. Just hold that thing,'" Yon said.

The former special ops weapons specialist called over an explosive ordinance disposal expert, who destroyed the item far away from the other weapons.

Caches like the one found in Mosul by Iraqi police in 2005 are nothing new for troops on patrol in Iraq, Yon said.

"You wouldn't believe it," he said. "It's as if [Saddam Hussein] took most of this oil money and the stuff he didn't build palaces with, he bought weapons."

Although a soldier destroyed this ordinance, Yon still wants to solve the riddle of "Mystery Weapon 2."

"Somebody's got to know, obviously, because it was clearly made in a factory somewhere," Yon said. "It's got to be in somebody's book."

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