In the final phase of the Tomb of the Ancients (outside the door to the Clark boss fight), there is a secret passage leading to a blocked-off area showing a campfire party with a dancing Grand Minimus, two baby Globoxes playing drums, and a large character named Jeff, a depiction of Ubisoft lead 3D programmer François Queinnec, playing guitar.

This secret was originally created by an unknown artist and developer Arnaud Guyon for the Nintendo 64 version of the game and can be accessed with the following combination: Hold L + R while pressing A/B/A/B/A/B. This will cause a crate to appear that can be used to enter the passage. When the PC version of the game was developed, this cheat was changed to standing in a specific spot in front of the passage and typing "playjeff" to make the crate appear. Guyon posted details of this variant of the Easter egg on his website in 2002, but it would go undiscovered by players until 2018.

This area can also be accessed in the Dreamcast and Nintendo 3DS versions of the game using the following combinations:
Dreamcast: Press A/B/X/Y/X/Y
Nintendo 3DS: Press B/Y/D-Pad Right/D-Pad Left/D-Pad Right/D-Pad Left

In the Nintendo DS, iOS and PlayStation 2 versions of the game, the hidden area is available, but can't be accessed, either due to the respective buttons not being available to press or because the cheat was disabled. In the PlayStation version, it does not appear at all.

In the PlayStation version of the game, collecting at least 720 Lum will unlock a secret 2D level. It is believed that this is a remaining level from the cancelled 2D sequel planned for the original Rayman.