It was a tooth from a 10,000-year-old mammoth.
Groundskeeper Patrick Walker found the 10-pound tooth last week on the golf course near Grand Rapids, Mich.
The mammoth tooth was exposed by heavy rains.
The recent high school graduate says he knew what it was, because he paid attention during science class.
Researchers investigating the site found other mammoth fossils.
They've decided to leave them in place for the time being.