Finding the densest not necessarily periodic sphere packing.
Kepler Problem
See also
Kepler Conjecture, Sphere PackingExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Kepler Problem." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/KeplerProblem.html