"pack" might have been a more appropriate term than "stack". Tightly packed circles (in 2d) form a hex grid. More relevant, though, might be the shapes you get

"pack" might have been a more appropriate term than "stack". Tightly packed circles (in 2d) form a hex grid. More relevant, though, might be the shapes you get when packing spheres: https://en.m.wikipedia.org/wiki/Rhombic_dodecahedral_honeycomb https://en.m.wikipedia.org/wiki/Rhombic_dodecahedral_honeyco...