Taylor Wood, 17, of Hooper, said he was mowing just prior to 1 p.m. Sunday when he heard a small object beneath the lawn mower, the Ogden (Utah) Standard-Examiner reported Thursday.
"I heard a thud," Wood said. "I stopped and looked down and read on it. I saw real clear 'TNT shell' on the side."
TNT, while an explosive, is very stable and resistant to shock and friction. It's the common measure for the explosive power of other substances, including nuclear bombs.
Charise Wood, the teenager's mother, said the object was the same approximate size and shape as a small can of tomato paste.
Capt. Krista Horting of Weber County Fire Station 63 picked up the canister and gave it to the Weber County Sheriff's Department.
"It was a very small device," she said. "It was pretty contained, no openings anywhere."
The sheriff's office said the canister would be taken to Hill Air Force Base for disposal.