The Catacombs is the largest communal cemetery in the Early Christian Age in Milos. Three large and independent subterranean galleries (Α,Β,Γ), carved in the porous, volcanic rock, formed the initial structure, and each gallery was connected with other, smaller ones. Today, tree galleries form a single complex through artificial passages constructed in the 20^th century. A rectangular funeral chamber is the form of the cubicula of Rome's catacombs completes the complex.

Gallery width varies between 1 and 5 m., and so does galley height (1,6 to 2,5 m.). The walls have been carved to form arched recesses, arcosolia, for tombs, while a considerable number of graves are found in the ground of all the galleries. Certain arcosolia, have painted decoration with their border coloured usually red and their drum painted with a deep blue colour. Only traces of this decoration have survived, as well as a few fragments of inscriptions. These few inscriptions are significant for the information they provide to us concerning the names of Christians, the ranks of the ancient clergy and the belief of the first Christians to the angels, as patrons of the tombs.

The present main entrance to the catacombs has been opened at the beginning of this century. Hence, the visitor enters the second gallery (B), witch is the only one one can visit at the time. On the right side of galley Β survives the only "two - leveled" tomb of the catacombs as well as fragments of a very significant inscription, written with capital red letters within a rectangular frame.

The catacombs, besides been a cemetery, were a place of worship, as this indicated by a piece of rock witch is left at the middle of galley Β, so as to be used as altar. During the 1928 excavational research, the traces of bases found at the four corners of the roof, in correspondence to those revealed on the ground, suggest that originally over the altar was probably a ciborium. The figure that resembles a sarcophagus led to the assumption that this was the tomb of an official or of one of the first bishops of Christian Melos.

Galley Α, to the west of the present central gallery, is distinguished for its width, its general spaciousness and for the double family tombs, witch are carved in its walls. The entrance to the south side is today obstructed. Gallery C, to the east of the central gallery, survives only partially, as its south part, witch had the original entrance, has now collapsed.