FLINT, Mich.—Officials in Flint, Mich., say they’ve had to replace hundreds of manhole covers and grates that were probably stolen and sold for scrap.

The Flint Journal reported Monday that nearly 400 cast iron covers and grates have been taken from streets in the past year. A cover can fetch $20 from a scrap yard but can cost the city more than $200 to replace.

Officials in neighboring Burton say they’ve lost about 200 covers and grates during the same period. Utilities supervisor Mike Holzer says it leaves behind holes up to 35 feet deep.

Genesee County officials say they’ve been able to reduce thefts of county-owned covers by outfitting them with a bolt that is turned by a wrench only they have.