Bump in the Night

Old School Thoughts on Darkness, Delving and DM-ing.
Classic, White Box D&D can be described as a treasure hunter's game due to its mechanical emphasis on combat and the accumulation of treasure. The game has its roots in the tabletop wargame where simulated combat is the focus of play, but adds additional elements which make it more of a character driven story-like experience. The players learn of a potential treasure, perhaps guarded by a monster or lost in some ruined underground caverns and set out on an adventure to reclaim this legendary treasure. (I am thinking of Jason and the Argonauts and other Greek myths.)
The dungeon is often associated with early D&D and this is obviously not surprising given that its creators, Dave Arneson and Gary Gygax both ran their early campaigns centered on exploration of an underground dungeon of many layers (levels in game terminology). Dark underground halls where danger awaits and exciting rewards can be found offered many hours of play. The game encourages repeat visits to the underground as the accumulation of treasure and experience points leads to the characters leveling-up therefore enabling the now more powerful characters to delve into deeper dungeon levels where even greater monsters and treasures await.
In the original game rules, characters, even dwarves and elves, have normal human-like vision and are therefore reliant upon a source of light in order to see what is in front of them. Monsters who inhabit the underground possess a kind of "darkvision" giving them an advantage in their unlit environment.
The underground is a magical otherworldly place where strange things are common. This is part of the appeal - to enter an alien unknown world where dangerous and wonderful adventures take place, and to survive and return to the relative safety of the surface world where things are more normal. (It much resembles the classical journey of the mythical hero.)
It is the referee's job to describe this imaginary underworld and its monstrous inhabitants. Drawing on imagination and using spoken words and sometimes pictures and even miniatures, the referee must bring this fantasy setting to life for their players. In some ways this is easier if the referee leaves aside the visual aids and describes what other senses perceive. An odor, a change in temperature, a sound, a breeze, a light touch, even a sixth sense awareness of something watching you...these can help set the mood and build suspense.
The flickering, shadowy light of a candle placed on the game table can add to the "scary" mood if the modern electric lights are turned down a bit. Maybe drop the thermostat a few degrees to create a bit of a chill in the room. Arrange for a timely sound, or some low ambient "mood" music in the background and you have set the table for some "creepy" monsters to go bump in the night. Perhaps there is a good reason so many D&D sessions were played in one's basement back in the day!