by Atari Program Exchange (APX)
You've been placed on a Martian moon named Phobos, and you're piloting a ship that plummets straight down through the surface as you seek and destroy renegade Martians who've built bases there. Other targets include fuel tanks and deadly saucers and missiles. Smashing into anything on the screen is sudden death, but maneuverability is excellent.
A total of sixteen caverns are waiting to be explored, and it gets more difficult to squeeze through the tricky cavern entrances as you zoom deeper beneath the surface. At the very bottom, you'll get a crack at the Martian Headquarters. Knock it out and those industrious Martians will rebuild everything instantly, so you have to start from the top again. Current and high scores are constantly posted. Color is outstanding and is superior to Caverns of Mars. Sound effects are well defined and do the job. Four modes of difficulty extend's the game's longevity.
It's amazing how good this game is when you consider that Greg Christensen was merely a normal Atari user when he created Caverns of Mars and Phobos. This is yet another excellent example of the many great user-written games that were made available - and possible - by the Atari Program Exchange.