That's actually an artistic effect. The black should properly start at 3280ft (1km) of depth. Below 200m only about 1% of light penetrates, below 1000m it's pitch black (darker than the darkest dark most humans have ever experienced in their lives).

I think you're underestimating the ubiquity of caves.

Or a darkroom. Or just a closed closet in a dark house on a moonless night. Black is black, it's hardly beyond most of our experience to be sightless.

I assume it's as dark as being deep down in cave? It's much easier to experience that, and I'd say quite a few people have.

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.

Same for me on every cave trip. The fun part is that if you do it long enough your brain starts making up shadow images that you would swear are real.

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.

Very few people have ever gone into a closet and closed the door?

I had a very similar reaction as I scrolled down, somewhere between creepy and despair.

That's because the deeper you go, the closer you get to Hell, which everybody knows is at the exact center of the Earth: http://www.orthodoxengland.org.uk/demonsscr.htm

Wasn't the earth considered flat when hell became a concept? How could it have a center?

I don't know. The Greeks figured out the earth was spherical, but maybe everyone before them thought is was flat. Go ask the voices in the hole in Kola.

Thank you for pointing out the obvious with a reference to Snopes.