A turner's cube is a traditional machinist's exercise to test consistency and tolerances when milling metal. But this two-millimeter cube in a cube in a cube in a cube in a cube is next-level skill.
He says "I etched it after machining to remove the burrs and to give it a nice surface finish." He also shows that if properly milled, they spin perfectly like a top.
Here's the regular-size turner's cube people usually create: