The route along the freeway in Boston that has been relocated underground (the Big Dig) is to become a greenbelt. As the old freeway is dismantled, Robert Campbell is taken with how big the space is. “It’s just possible that the greenway site is more stunning today than it will ever be again. That’s because of the way the remaining fragments of the old artery stand around in it, like mysterious wreckage from an earlier civilization. They may be junk, but — as artists long ago discovered — junk can be visually rich.”