The Quake engine is a game engine created for the 1996's Quake. The majority of the programming was done by John Carmack, with help from Michael Abrash. Sometimes referred to as "id Tech 1.5", much of the engine code was carried over to id Tech 2 and id Tech 3.
In December of 1999, John Carmack released the source code of the engine onto the internet under the GNU General Public License. This allowed programmers to create their own splinter versions of the engine at no cost. These often add additional features such as realtime lighting and shadowing or a performance boost. One of these splinter engines was Geo Mod, made by Volition for Red Faction, featuring realtime destructible enviroments.