hard drives storing the project's source code crashed, destroying it. Concurrently, backup tapes storing the code were also destroyed. This has been cited by id Software as one of the reasons why it republished the source code under a free license, as it believed that incidents like that could be prevented by requiring developers to share their changes. In April 2010, however, Lewis rediscovered the glDoom sources in one of his deceased friend's hard drives. Boom and derivatives Boom was a port for DOS of the Doom source code by TeamTNT. Boom fixed numerous software glitches and added numerous other software enhancements into the engine to such a degree that its additions have been incorporated into most modern versions of Doom source ports (such as PrBoom+, ZDoom and Doom Legacy). The last update of Boom was released on October 22, 1998. In October 1999, Boom's source code was released. Further development of Boom as a source port was continued for DOS as MBF, for Windows as PrBoom, and for Linux as LxDoom. The latter two later merged as PrBoom and also took on many of the MBF features, so PrBoom's own successor, PrBoom+, is effectively the modern equivalent of Boom. Eternity Engine The Eternity Engine is a Windows source port licensed under the GNU General Public License. It was first released on January 8, 2001, as version 3.29 beta 1. It was originally meant to power a Doom total conversion, but after that project went on hiatus (eventually being cancelled in 2006), the engine became the prime focus. The engine is based on Smack My Marine Up (SMMU). It includes such features as scripting, portals, polyobjects, and Heretic support. Marine's Best Friend Marine's Best Friend (MBF) is a DOS-based source port. It is based on Boom, and adds several new features including high resolution graphics, enhanced monster AI, emulation of the pre-release beta versions of Doom, and "helpers" that follow and help the player (specifically dogs, to which the name of the engine refers). It was developed by Lee Killough and is no longer updated. Its code was later used as the base of the source port Smack My Marine Up, which in turn was used to construct the Eternity Engine. Some of its code was also adopted in PrBoom. In August 2004, James Haley and Steven McGranahan ported Marine's Best Friend to Windows as WinMBF. WinMBF was last updated