The Army is spending about $3.5 million over nearly two months to search for unexploded ordnance off Makua Beach after a 5-inch projectile was found offshore in 2016, officials said.

The project, which started Monday, is expected to run to about May 23 and will involve contracted divers who are experts in underwater munitions, the Army said.

The 22-acre offshore area is centered around the spot where a local diver found the 5-inch round in 2016. The munition was recovered and destroyed by the Navy.

Use of Makua Military Reservation for a firing range by the Army and other U.S. military services dated back to the 1920s, and the location where the ordnance was found is within the vicinity of the range, the Army said.

“That’s the only thing that’s been found out there” in that offshore spot, said Dennis Drake, a spokesman for U.S. Army Garrison Hawaii.

Excess World War II munitions were also dumped by the military in such a quantity over a 5-square-mile area off Pokai Bay that it became known as “Ordnance Reef.”

The search spot off Makua Beach is 1,300 feet across and runs up to 765 feet into the ocean, Drake said. Divers will work in three-person teams using metal detectors and will swim in defined patterns along the ocean floor, according to the Army.

The search is being conducted on weekdays. The “exclusion zone” will be marked with buoys and closed to boating, swimming and diving, the Army said.

Officials also said a small portion of the beach will be closed to accommodate support equipment and dive operations.

If underwater unexploded ordnance is found during the search and is safe to move, it will be brought to shore for destruction in a designated area on Army property, the service said.

In that event, the Army said it will coordinate with the state Department of Land and Natural Resources and Honolulu police to establish a safety zone that might require the closure of Farrington Highway adjacent to Makua Beach for no more than 20 minutes at a time.

The Army said it had to wait for federal unexploded ordnance removal funding after the 5-inch round was found, which caused a delay. The project was originally scheduled to begin in October, but a fully operational and staffed hyperbaric chamber required by safety regulations for the divers was not then available.

The Army said in 2011 that a recovery effort at Ordnance Reef resulted in the removal of approximately 2,380 small, medium and large items that appeared to be munitions. A total of 331 pounds of explosives was destroyed, the Army said.