Mathematicians prove 400-year-old 'stacking problem'
A team led by mathematician Thomas Hales has delivered a formal proof of the Kepler Conjecture, a 400-year-old geometry problem explaining the most efficient way to stack equally-sized spheres in a 3D space. In 1998, Hales had announced completing a 250-page proof compiled with 3GB of data files, but verification by referees took another 19 years to publish the proof.