The guides turned the lights out in a cave I went in down in Venezuela to show us what pitch black really is. Kinda creepy esp with the bats, birds(Guacharo birds that use echo location), muddy floor littered with dead animal carcasses, centipedes, rats, dank smell ect. We were about a mile into the cave when they turned the lights off.

Half of communication is listening. You can't listen with your mouth.