After literally wandering around in the darkness and just by a stroke of luck escaping plunging into the terrible pit that lies in the middle of his cell, the unnamed narrator sleeps, is given food, and then sleeps again through the drugged victuals that he is provided with. When he wakes up again, he finds his cell lit by a "wild, sulfurous luster," which allows him to see his cell and also to find out whether his initial thoughts and assessments of his prison were correct or not. Note what he discovered:
In its size I had been greatly mistaken. The whole circuit of its walls did not exceed twenty five yeards... I had been deceived, too in respect to the shape of the enclosure. In feeling my way I had found many angles and thus deduced an idea of great irregularity; so potent is the effect of total darkness upon one arousing from lethargy or sleep! The angles were simply those of a few slight depressions, or niches, at odd intervals. The general shape of the prison was square.
In addition to having two of his central deductions about the size and shape of the prison proved wrong, the narrator sees that the walls of this prison were made of some kind of metal, covered in frightening shapes and images. The narrator also sees how in the centre of the cell "yawned the circular pit from whose jaws I had escaped."