You can unlock three secret levels in the game:
Young Indiana Jones - to unlock this level you need to find all artifacts from the Raiders of the Lost Arc campaign (or collect 6 artifacts).
Ancient City level - To unlock this level you need to collect all artifacts from The Temple of Doom campaign (or collect 12 artifacts).
Warehouse - To unlock this level you need to collect all artifacts from The Last Crusade campaign (or find all 18 artifacts).
When you unlock certain level, you can start it in the Artifact Room in the University.