Computer improved packing of 423 circles in a square from this:

http://hydra.nat.uni-magdeburg.de/packing/csq/csq423.html

to this:

http://www.algit.eu/htmlji/Packntile/Packing_Contest_01052010/SQU_cir_423_pic.html

An innovative horizontal fracture enables a slightly denser packing, was of course found by this "packing AI".

(Eventually the links will show the same configurations, since we submit our results to Eckard Specht packomania's owner, who provides some additional details to the solution.)