This vast complex of galleries cut into the volcanic rock is laid out over two levels, forming a "baptistery" and a spacious basilica with three aisles (4C-6C). The tomb of St Januarius, whose remains were transported here in the 6C and remained there until they were removed in 831, is decorated with frescoes depicting the saint. Note the funerary niches decorated with beautiful paintings (3C-10C) and, in particular, on the upper level, the first images of St Janvier (5C).