The further we go, the stranger it gets.

Exploration Stage: Artifact Bank
From: Mr. Black
Description: Get away from the vault, you're about to trample it down if there were any traces left behind. Let me examine it with my "green outer wire" loupe. It's a special wavelength of light, a kind of greenish ultraviolet. There's a footprint! Let's check it out.
Completed: Hmm, this is part of a footprint. They have not been erased, but have simply disappeared, as if they suddenly stepped into the void. This must be another riddle by the guardian. Was this wall built after that something left its footprints? Maybe there is an invisible door here.