The analogous configuration in three dimensions, with spheres replacing the coins, has puzzled mathematicians for more than three hundred years. The great German astronomer Johannes Kepler (1571– 1630) conjectured in 1611 that this arrangement represents the most efficient packing of identical spheres, resulting in a density of π
/1 8 spheres per unit volume, or slightly more than 74 percent. Amazingly, his conjecture—well known to any fruit vendor who packs a pile of oranges in a box—remained unproved until 1998, when the American mathematician Thomas Hales used a computer program to exhaust the large number of possible cases.