Detroiters are watching their steps these days because of a rash of thefts of manhole covers.

In the past three months, a city official said, sewer scavengers have stolen 400 to 500 manhole covers, leaving dangerous gaping holes behind.About 40 of the cast-iron lids were stolen in a single day recently in a six-block area on Detroit's west side, said James Heath of the Detroit Water and Sewerage Department.

The manhole covers weigh 112 pounds or 240 pounds, depending on the model, and cost from $35 to $50 to replace.

Officials believe the lids are being sold to scrap yards, which sell them for scrap for about $10 to $15 each or less.

Heath said some thieves have used winches but others, masquerading as construction crews, have used their bare hands.

Although no serious injuries have been reported, the recent thefts prompted the police department this week to order patrol officers to be on the lookout for thieves.

Executive Deputy Police Chief James Bannon also is asking the public for help in finding the thieves, whose trucks ride low from all the weight.