1 Monty: Dad, grandad, Dr Smith - get down here and have a look at this!
2 Monty: There's a thin layer of something. It sounds hollow underneath.
2 [sound]: Tap tap...
3 Minnesota Jones: Ummm. This looks like firmament.
3 Monty: Firmament? But that would imply...
4 [sound]: CRAAACK!
4 All: Aaaaaaaa!!

I really need to stop looking in Wikipedia for references I can link to. I just spent half an hour perusing ever less-closely related pages to the one I originally intended to link here.

See if you can navigate your way back from here: Lichenometry.

---

2021-11-07 Rerun commentary: The page you're trying to get back to from Lichenometry is Firmament, of course. It's much easier if you start there and try to wikilink-walk your way forwards to Lichenometry.

I just checked that the path is still there, and yeah, it's actually not that difficult to find with a little logical thought.