Penumbra is real - possible inspirations?

[Void]

If you happen to visit Germany, you can take guided tours to many of its Penumbra-esque underground places i.e. under the cities of Berlin or Hamburg:

• Berliner Unterwelten e.V. (engl. "Berlin Underworlds Association")
  
• Unter Deutschland (engl. "Under Germany", including Unter Berlin & Unter Hamburg, website in german only)