Although Gauss solved one version, it was still part of Hilbert's 18th unsolved problem.  
Although Gauss solved one version, it was still part of Hilbert's 18th unsolved problem.  While it's thought that Gauss's result of pi over the square root of 18 applies to non-lattice packings, only recently has Thomas Hales claimed he's shown the face-centered cubic lattice to be the most efficient arrangement to stack spheres.  FTP, name this conjecture created in 1611 by a famous astronomer.
Although Gauss solved one version, it was still part of Hilbert's 18th unsolved problem.  
Kepler conjecture