The nave is formed by a line of seven columns in their original places, in a wall of _débris_ built to support the church above. These columns are of beautiful marbles, and stand upon a wall of the imperial period, which has been traced for 98 feet. At the west end of the north aisle a flight of narrow steps leads down to a passage, 25 inches wide, formed between massive walls: that on the right is brick of the imperial period, forming the wall of S. Clement's house; that on the left, tufa, of the kingly period, being part of the walls of Servius Tullius. This has been heightened by a travertine wall of the republican period. The tufa wall has been traced for 500 feet, and the travertine wall upon it for 410 feet. About 20 feet is still buried, showing how low ancient Rome was in this valley. At the end of this passage another flight of steps leads up into the south aisle. In the centre of the passage is an entrance through the imperial wall (now blocked up on account of the water) into THE ORATORY OF S. CLEMENT, reached from the south aisle by a broad flight of twenty steps. The Roman Catholic Church has faithfully handed down the tradition that S. Clement erected an oratory in his own house, between the Cœlian and Esquiline Hills, which must have been built, as we have seen, close to the walls of the city--a not unusual thing as the city grew. Several chambers remain to be excavated at some future time. A long passage has been cleared out, in which was found a doorway bricked up. This was broken through, and found to be a TEMPLE OF MITHRAS,