The narrator of Edgar Allan Poe's "The Pit and the Pendulum" is a prisoner who is being tortured and tormented as part of the Spanish Inquisition. He tells us that he is in a room, in the dark, and he is obviously being watched by his inquisitors. His thinking is fuzzy and unclear, but he is able to describe his condition and his surroundings well enough for us to know he is in trouble and he is being watched.
He is awake and then he passes out again several times; eventually he is awake enough to try to measure the dimensions of his cell by groping his way along the wall. He is not able to use a particularly scientific method, but he does manage to work his way around the perimeter--or so he thinks. The next thing he decides to do is walk across the cell or room.
At first I proceeded with extreme caution, for the floor although seemingly of solid material was treacherous with slime. At length, however, I took courage and did not hesitate to step firmly--endeavouring to cross in as direct a line as possible. I had advanced some ten or twelve paces in this manner, when the remnant of the torn hem of my robe became entangled between my legs. I stepped on it, and fell violently on my face.
It takes him a bit to realize something significant, but eventually he does: while his chin is on the floor, the rest of his face hanging over a ledge and is touching nothing.
At the same time, my forehead seemed bathed in a clammy vapour, and the peculiar smell of decayed fungus arose to my nostrils. I put forward my arm, and shuddered to find that I had fallen at the very brink of a circular pit....
It is still black as midnight in the room and he has no way of knowing what is in the pit or how far down it goes; however, he drops a little piece of cement down the pit and it falls for a very long time until it finally lands with a splash into some water and then echoes.
If he had not tripped and fallen on his face, he would have walked right into the pit and been gone forever. As it is, the narrator hears his inquisitors react to his fall. Obviously they are disappointed that he is still alive, and the next time their prisoner is sleeping, they begin a new torture--the pendulum.