Internally Carved Acrylics
A variety of grinding and cutting bits including those that are made specifically for this unusual art are used with a rototool like the Dremmel brand onto the rear surface and into cell cast acrylic to create interesting three dimensional effects.

This view from the side shows the extent of the internal carving of rose petals made with a special helical cone bit. The thickness of the acrylic piece is 3/4" and some of the petals' extremeties reach a good way through the entire thickness of the piece.