Mirror Blocks is a 3x3x3 cube modification where each of the three layers on each axis has a different thickness. It is usually unstickered or stickered with only one color, so that rather than solving it based on the colors of each piece, the solver must go off of the shape of each piece, to return the puzzle to its solved, cubic state.
Originally known as the Bump Cube, it was invented by Hidetoshi Takeji, and appeared at the design competition during IPP26 (International Puzzle Party 26) in Boston in 2006. Rubik's and MegaHouse then mass-produced the puzzle starting in September 2008. That mass-produced version was renamed Mirror Blocks.