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.
The Evening Star or 'Pig Iron Wreck' appears to be the remains of a tanker that ran aground and broke up. The wreck sits in 40 ft of water and is scattered over a relatively large area.