The whole thing got me thinking on how it could, possibly, be added to the underground areas. So here it goes:
-Fungi based weather/time measurement. Add a species of fungi throughout the subterranean areas (though not in all of them), that releases its spores during certain seasons of the year. Make it bioluminescent, with the cycle of light/Dark marking the passage of day.
-Add a behemoth of a beast (something like a stoneskinned bullete three times the normal size) that ever so slowly migrates through a known path, passing through each spot only once a year. Make it something with a gajilion hp.
-Caves that flood/are revealed with the tides.
-Migrating Rothe Herds.
-Random Earthquakes that cause random (bashable,perhaps even minable) rockslides.
And that is it.
EDIT: Reaction to Mithreas' reply:
