Caverns of Mars is a vertically scrolling shooter for the Atari 8-bit family of home computers. It was programmed by Greg Christensen, with some features added by Richard Watts, and published by the Atari Program Exchange (APX) in 1981. Caverns of Mars became the best selling APX title of all-time and was moved into Atari, Inc.'s official product line, first on diskette and later on cartridge. The game is a vertically scrolling variation of Konami's 1981 arcade game Scramble. In Caverns of Mars, the player descends into cave and at the end must retrace their steps back to the top. Christensen wrote two less successful sequels, one of which scrolls horizontally and is very similar to Scramble. Gameplay Caverns of Mars is a scrolling shooter similar in concept and visual style to the 1981 arcade game Scramble. Christensen changed the orientation of the caverns from Scramble, having the player fly down into them as opposed to sideways through them. Unlike Scramble, rockets in Caverns of Mars remain on the ground. Using a joystick, the player controls a ship descending into the tunnels of Mars, firing at targets along the way. The player's spacecraft has two cannons, positioned on either side of the craft, firing downwards. The player needs to avoid hitting the cavern walls, while shooting targets of opportunity along the way. Fuel tanks can be shot to add 5 points of fuel, and the craft is destroyed if it runs out. There are several different sections of the map, with easier skill levels removing the more difficult sections from the areas through which the player has to fly. The easiest skill level has only three sections, the hardest has six. On any skill level the last section of the map is a reactor, which the player lands on and thereby sets to explode. The player then has to reverse course and fly up and out of the caverns to escape before the reactor explodes. Development Greg Christensen, a high-school senior, purchased an Atari 800 in 1981, and created Caverns in "little more than a month and a half"—the first significant program he wrote in 6502 assembly language. Fred Thorlin of Atari Program Exchange recalled Caverns arriving at APX: In the original version, when the player reaches the end of the selected map, the game ends. Thorlin felt it needed something more. Christensen was too busy, but agreed use his