Raumschach
The inventor contended that for chess to be more like modern warfare, attack should be possible not only from a two-dimensional plane but also from above (air) and below (underwater). Maack's original formulation was for an 8×8×8 board, but after experimenting with smaller boards eventually settled on 5×5×5 as best. Other obvious differences from standard chess include two additional pawns per player, and a special piece (two per player) named unicorn.
Read more about this topic: Three-dimensional Chess