Chicago Pile One - Level 10
Tenth layer of graphite blocks containing pseudospheres of black and brown uranium oxide. The brown briquets, slightly richer in uranium, were concentrated in the central area. In the foreground and on either side are cavities filled with graphite, now presumed to have been an expedient measure dictated by shortage of fuel and, possibly, a last minute change in the lattice arrangement.
Note the hammer toward the bottom of the photo.