Working on an orcish language, I'm finding the fact that they spend much
(most) of their time underground to be presenting some interesting

Specifically Time.
What natural cyclical phenomena are there that operate detectably in an
underground environment?

I've got the bat-flight/bat-roost cycle, and was wondering about tides, but
don't you need a pretty large body of open water for that to be an
appreciable factor?  IIRC, lakes don't get appreciable tides.  Is there
anything else I could use?

And what about longer time periods?  The "year" is presumably more or less
irrelevant if you're down deep enough- would their longer time units be
just arbitrary, and probably varying widely by tribe?

The other main challenge is creating a subterranean ecosystem big enough
and diverse enough to support humanoids, but as a biology graduate, I'm on
more or less home turf there...
The one problem is sources of energy.
I've got tree roots, bat (& other) guano, and bacteria around volcanic
vents and subterranean geysers/mudpools.  Have I missed any other ways that
energy can enter the system nonmagically?