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 two interconnected portals that can be placed on any non-metallic surface (wall, ceiling, or floor). Co-founder of Valve, Gabe Newell, took interest in the team's work and immediately gave them jobs at Valve. The developers went on to write the critically acclaimed Portal using many of the same concepts.