Narbacular Drop is an environmental puzzle video game developed by Nuclear Monkey Software. It was released for free online in 2005 for Microsoft Windows. It was the senior game project of students attending DigiPen Institute of Technology. The gameplay consists of navigating a dungeon using an innovative portal system. The player controls a pair of interconnected portals that can be placed on any non-metallic surface (wall, ceiling, or floor). Gabe Newell, managing director of Valve Corporation, took interest in the team's work and employed the whole staff at Valve. The developers went on to make Portal (2007) using many of the same concepts. The word Narbacular, which does not exist in any dictionary, was chosen primarily to aid in internet search engine results. Being mostly a proof of applied concept, the game contains only six puzzles to solve. However, members of the Narbacular Drop forum community have created a catalog of custom maps.