The workers called police earlier this week after finding the skull on Spencer Place, Police Chief Jacqueline Luthcke said.
Forensic scientists will need to determine its age, which looks to be at least 10 years "and could be closer to 100," Luthcke said.
How it got there no one knows, but there doesn't initially seem to be any signs of foul play, the chief said.
"There are many theories on what could have happened, but we're unable to know for sure right now," she said.
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