This 1935 cover shows three workers in a sewer tunnel in Chicago that is being driven through blue clay. The article states that the excavation “is done by a method seldom seen elsewhere. This material cannot be handled by any kind of a mucking or excavating machine but must be sliced off faces and benches by U-shaped knives operated by hand or a hoist line…. The face of the heading is worked vertically, usually with two men drawing the knife, a U-shaped blade with two handles, from top to bottom, shaving off a vertical slice. As the slice or slab of clay is cut loose, two muckers stand by to grab it and cast it back to more muckers, who rehandle it into waiting cars… This is back-breaking work, as the clay weighs about 165 lb per cu ft.”