All manner of concrete, steel, and stone rubble from dredging, demolition projects, and other construction is used as artificial reef materials. This material is generally available at very low cost or free from construction companies who are more than happy to get rid of it. Transportation costs determine where this material is used by the Reef Program.
Deep and dark, the Mud Hole is the Hudson River's channel from a time when the oceans were much lower. Today it collects all the silt and sediment that the river carries out to the sea, making it a very fertile fishing ground, frequented by pelagic fishes and sharks. The contour shown on the chart is not any particular depth, but do give an idea of the location.