The 19th layer of graphite covering layer 18 containing slugs of uranium oxide. Parts of the wooden scaffolding and surrounding balloon cloth also are visible.

The completed pile contained 771,000 pounds of graphite, 80,590 pounds of uranium oxide and 12,400 pounds of uranium metal when it went critical. It cost about $2.7 million to produce and build. The pile took the form of a flattened ellipsoid which measured 25 feet wide and 20 feet high.