Hilbert curve is a continuous fractal space filling curve, which was first described by German mathematician David Hilbert in 1891. Below you can see 5 first steps of the plane Hilbert curve.
But the Hilbert curve looks more interesting if it represent in three dimensions. Carlo H. Séquin, a professor of Berkley, created a small 5" metal sculpture of Hilbert curve, which he called "Hilbert 512". You can see it below.
It was also be modeled by Torolf Sauermann in the program Maxwell renderer. Below, you can see the second step of the three-dimensional Hilbert curve and two versions of cubic Hilbert curve.
 |  |
