Caverns of Mars was a very influential game in the development of the vertically scrolling shoot ’em up genre.
First released in 1981 through Atari’s APX (Atari Program Exchange) scheme, where professional and amateur developers alike could submit their work for consideration to be published by Atari, Caverns of Mars became so popular that it was added to the company’s “official” lineup of first-party releases.
The game was essentially a vertically scrolling take on Scramble, with the twist that you had to retrace your steps back up the cavern once you reached the bottom — easier said than done. It spawned a sequel that didn’t get officially released until Antic Software picked it up several years later, renaming it to Mars Mission II, and an APX-released spinoff called Phobos that isn’t nearly as well-known.
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