Yes. The only illumination would come from bio-luminescence or artificial light. Though a lot people have been in enclosed areas that are dark, very few people have completely turned off every source of illumination in such situations.
I obviously can't speak for anybody else's experiences, but every time I've gone into a cave, the group has spent several minutes in the pitch dark, being quiet. I can't really imagine anybody not doing that; going into a cave without experiencing the dark and the silence would be a much less interesting experience.
Our minds are not adapted to sensory deprivation. Hence, prolonged sensory deprivation can induce hallucinations, and exceedingly long sensory deprivation (combined with restraints to prevent movement) can impact sanity.
