Geoff Horswood <[log in to unmask]> writes:
> Working on an orcish language, I'm finding the fact that they spend much
> (most) of their time underground to be presenting some interesting
> challenges.
> Specifically Time.
> What natural cyclical phenomena are there that operate detectably in an
> underground environment?

Growth of stalacmites? :-)  Maybe a little slow and I suppose the speed
is subject to changes due to different amounts of water.

Something quite stable: a large pendulum (->Foucault) would work for
measuring planet rotation.  This, however, maybe too artificial as the
effect will not be very overt in daily life, so a pre-industrial
culture will probably not find this out easily.