Steel columns salvaged from the rubble of the World Trade Center are heading back to ground zero.
Two 70-foot columns taken from the wreckage of the north tower are being permanently installed Tuesday at the entrance to the Sept. 11 museum.
The 50-ton columns have three prongs and are known as tridents. They made up the facade at the tower's entrance. For the past few years they've been kept at a hanger at John F. Kennedy International Airport with other material recovered from the destroyed World Trade Center site.
The $45 million museum is being built below the 8-acre memorial plaza. It's set to open next year.