Remember that the narrator is relating this tale after it has happened. He recalls the "tall figures" taking him down into something. He adds that it is like an "interminableness of descent." The descent is completely silent and this adds to his horror and uncertainty. When his carriers reach the bottom, he struggles to regain his senses. He recalls his trial and sentencing (which happened prior to his descent). He then realizes he is lying on his back. Then he finally opens his eyes, but is unable to see anything because he is in complete darkness. He can only guess if he is dead or in some dungeon, awaiting his eventual execution. He has been carried into a subterranean area with a pit in the middle of it. He lies on the perimeter of the pit in total darkness. Evidently, this first attempt at his execution will succeed if he falls into the pit. He spends what little energy he has trying to determine the dimensions of the room/cave he is in. He miraculously avoids the pit, only to be taken to the next attempt at his execution: the pendulum.