Indiana Jones and the Infernal Machine Easter Egg - "Ancient Machinery"

In the Palawan Volcano, there is an area with a lava pit and a large statue. To the right of the pit is a large gong you can crawl under. Pressing one of the walls in the small passage behind the gong will cause a stone carving of an N64 to appear above Indy's head, and he'll say "Ancient machinery! It's worth a P.H.D to anyone who can figure it out!"

It's very secret (I only found it by complete chance), and it must have had some personal significance to someone, or they wouldn't have put it in there or bothered to make Indy say anything.