According to a report from the Associated Press, the operator went to police after his excavator bucket struck metal on a jobsite in Innsbruck June 18.
Police investigated the jobsite and realized that the operator had unearthed some 400 live grenades from World War II.
The discovery led police to evacuate all apartments close to the find, “while demining experts defused some of the ordnance and blew up the rest,” the AP reports.
An underground ammunition depot is the likely source of the grenades, officers said.