from the cover:
This is an unofficial surface world adventure of The Hollow Earth Expedition Roleplaying Game. It can be used as a one shot adventure or as part of an ongoing campaign. This adventure requires at least on character to have Academics religion or occult. Other useful skills include Linguistics Code and deciphering. This is due to the large amount of puzzles found within.
from Plot Summary:
The Mithraic Mysteries was a mystery religion practiced throughout the Roman Empire. The name Mithras was adopted from the Persian god Mithra. Worshipers of Mithras had a complex initiation system that consisted of several grades. They met in underground temples, which is where this adventure takes place. The Temple of Mithras in London is a real temple. The players explore the temple and if they can rise to the challenge receive the light of truth from Mithras himself. The “treasure” is left to the individual Game master. Anything can be found behind the massive stone doors including ancient text, treasure, or other plot items.