Glass reliefs 
It is possible to simulate a three-dimensional relief via stacked glass layers. Each glass layer, often consisting of a transparent plastic sheet, corresponds to one elevation level. Several layers stacked vertically can represent a three-dimensional object. All layers together form a transparent block and the features depicted seem to be suspended. The steps originating from the discrete layers cannot be eliminated. In such a manner, it is possible to easily create reliefs derived from contour maps by gluing maps on the layers: For each contour level, a map cut along the corresponding contour is glued on a new layer. Instead of using maps it is also possible to draw directly onto the glass.
This technique allows depicting even tiny details like the framework of a latticework bridge as well as caves, mines or tunnels.
 This article bases on Terrain models and relief map making. 1956. p.53-62. and Mühle, Helmut. Manuelle Geländemodell-Herstellungsverfahren unter Berücksichtigung des Standes der Technik. 1963. S.44,45,53.