level 0  +-------+  (4 objects in the node, recurse)
         |* *  * |
         |       |
         |*      |
         +-------+

level 1  +---+---+ (2 objects in NW node, recurse)
         |* *| * |
         +---+---+
         |*  |   |
         +---+---+

level 2  +-+-+---+ (1 object in each node, stop recursing)
         |*|*| * |
         +-+-+---+
         |*  |   |
         +---+---+