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).
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.
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.
"Sperm whales are believed to be able to reach 3 kilometres (1.9 mi) and remain submerged for 90 minutes." - that's a lot of darkness for an air breathing mammal to go through.
On a related subject: why is the blue whale referenced on the chart? It's big, but doesn't dive particularly deeply. It eats krill, which themselves are tied to photosynthesizing plankton which are tied to the surface waters. There's no reason for it to be interested in diving beyond there.
Perhaps because it is the biggest? That is really the only explanation I can think of. Unless you wanted to show where the plankton feeders hunt to illustrate light penetration.
