[myffical]

The Kelvin problem: what is the optimum way to partition 3D space into identical cells, such that the surface area between them is minimized? Lord Kelvin conjectured that the Kelvin cell gave this optimum partitioning. This conjecture stood for 100 years until Denis Weaire and Robert Phelan found a better cell in 1993.

http://en.wikipedia.org/wiki/Weaire%E2%80%93Phelan_structure