  Hilbert's two-dimensional space-filling curve is appreciated for its good
locality properties for many applications. However, it is not clear what is the
best way to generalize this curve to filling higher-dimensional spaces. We
argue that the properties that make Hilbert's curve unique in two dimensions,
are shared by 10694807 structurally different space-filling curves in three
dimensions. These include several curves that have, in some sense, better
locality properties than any generalized Hilbert curve that has been considered
in the literature before.

Computational Geometry