The famous hexagonal basin of Trajan was at the centre of many warehouses. The sides of the basin measure 357.77 metres, perhaps 1200 Roman feet. The maximum diameter is 715.54 metres. It was five metres deep. The bottom was covered with stones, at the north end gradually sloping upwards, to reach a depth of only one metre at the edge of the basin. On the quays were travertine blocks with holes, used for mooring, fifteen metres apart (h. 1.10, w. 0.75, d. 2.0, diam. of holes 0.45). The basin could contain more than 100 ships that did not moor alongside the quays, but at a straight angle.
The basin was surrounded by a few wide treads (total width c. 6 m.). Numbered columns with Latin numerals were also found around the basin, suggesting that the sides of the hexagon were subdivided into numbered sectors. On the quays was a wall, with five narrow doorways (1.80) on each side of the hexagon. The doorways are too narrow for wagons. Apparently the goods were unloaded and carried by slaves. This can also be seen on several reliefs and mosaics. The wall facilitated the control of the flow of goods, for the Customs Service and the levying of import duties (the portorium).
The following inscription (CIL XIV, 113) was found in 1794 on a quay on the south side of the hexagon:
COLVMNAM VII TEMPEST[atis]
It records the restoration by Septimius Severus in 196 AD of "a column broken by the force of a storm". The inscription seems to have belonged to the base of the column that had been restored. Perhaps this column can be seen on the Torlonia-relief, a famous Severan depiction of the harbour.
Side V of the hexagon with a white, travertine mooring block.
Testaguzza 1970, p. 170.
A marble column with the number XXIII at the corner of sides III and IV.
Testaguzza 1970, p. 171.