In the inner circle of Atlantis, after leaving the canal through the bronze doorway, there two markings near the burrowing machine (one on the inside of the machine's housing, and one on the outside). Look at either one first, and Indy will say, "Atlantean Graffiti." This is a reference to George Lucas's 1950's nostalgia movie American Graffiti. Look at the other marking, and Indy will say "More Atlantean graffiti." This is a reference to American Graffiti's sequel, More American Graffiti. These markings are also hints on how to operate the machine.