The genre's roots can be traced back to "Spacewar!", one of the earliest computer games, developed in 1962. The shoot 'em up genre was later established by the hit arcade game "Space Invaders", which popularised and set the general template for the genre in 1978, and the genre was then further developed by arcade hits such as "Asteroids" and "Galaxian" in 1979. Shoot 'em ups were popular throughout the 1980s and early 1990s. In the mid-1990s, shoot 'em ups became a niche genre based on design conventions established in the 1980s, and increasingly catered to specialist enthusiasts, particularly in Japan. "Bullet hell" games are a subgenre of shooters that features overwhelming numbers of enemy projectiles, often in visually impressive formations.