Based on the 1981 The Residents album of the same name, Mark of the Mole was an early pioneer in the music game genre.
Had it been released, it would have been one of the first games to be based off a rock album (Data Age's Journey Escape came out about the same time this prototype was being programmed). It's unknown what Atari's plans for the game and how much input the The Residents had in the final design. The overall concept and subject matter of the game seems like it would be hard to market to the general masses, especially if they were unfamiliar with the band itself. The game was never mentioned on any internal documentation, and the only known reference to Mark of the Mole comes from a short promotional video where a two second clip of the game is shown. When programmer Greg Easter left Atari the game was nearing completion, but still had some programming left to do.
First a line of music plays (one of the songs from the Residents' Mark of the Mole record). You are a mole who travels down into a cave and taps on walls with a hammer. Different parts of the cave make different musical notes and you have to find the next note you need to complete the line of music which was just played. You are building a song note by note and you have to remember the tone of the next note you need in order to get it right. Each time you play the caves are different, so you can't just remember where to go. The game actually teaches you what is called 'perfect pitch' in music - the ability to hear notes and know where they are on the staff.