TRAVERSE CITY, Mich. (AP) — Cargo shipping was at a standstill Tuesday on the Great Lakes as superstorm Sandy churned waves up to two stories high, forcing crews to take refuge in bays and harbors and raising concerns about an economic blow if the shutdown is prolonged.

The lakes are a bustling maritime highway for ships that haul bulk commodities such as iron ore, coal, limestone and grain. The massive vessels, some longer than three football fields, are built for punishment and accustomed to plowing through rough seas — even during the “gales of November” that in bygone days scuttled many a craft, including the legendary Edmund Fitzgerald nearly 37 years ago.