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.