Bubble Bobble is a puzzle-action game featuring the two Bubble Dragons, Bub and Bob. When it hit arcades in 1986 it introduced brand-new play mechanics, with players defeating enemies by trapping them in bubbles they blew and then popping them.
Although it is possible to beat the game just by clearing all 100 rounds, it also contains secret elements such as a “true ending” not obtainable in single-player mode, hidden commands that lead to secret rounds and a ton of different items. These elements were one of the game’s most important selling points.
Each round even has different air currents that are not visible, making bubbles rise in unpredictable patterns. Aspects like forcing players to understand and make use of these different patterns while playing helped make Bubble Bobble a revolutionary action game at the time.