Boulder I is a rock that sits upright on a cushion. Walking around the object, the viewer notices that the object had a hole on one side and is actually hollow. On display is therefore not a stone but a stone surface. Given that the eye can only perceive surfaces, when one observes the object from the front, it is fully identical to a boulder. On the interior a thin, grid-like steel framework supports the exterior shell. The size of the opening on the back of the rock corresponds to the dimensions of the milling head of the CNC laser cutter used to carve away the stone, leaving only its thin outer sheath.
Source:http://juliusvonbismarck.com/bank/index.php?/projects/boulder-i/