1036 m. long, with dimensions of 1,80 X 1,80 m., the tunnel goes through the mountain Kastron, at an axis North/West-South/East, 189 m. below the top of the mountain and 55 m. above the sea level. In its inner part, at a depth of 2-9 m., it is situated the channel carrying the water to the city, through clay tubes.
The tunnel is considered as one of the most important technical achievements of ancient world. It is hewn in the rock, with appropriate supports made of wood; the entrances were built with stones and bricks.
Near the entrances there are also cave-like rooms, evidently for the staff supervising the aqueduct. The most significant feature of the construction is that the workers started working from both sides of the mountain and achieved to meet at the middle, with a deviation of just 5m.!
The whole work was accomplished by Lesbian captives, under the direction and supervision of Eupalinos, an engineer from Megara (Attica); this latter had also constructed the aqueduct of his home city. (In his honor one the newly constructed tunnels of the National Road Athens -Corinth has been named «Eupalinos»).
It has been used as an aqueduct up to the end of Roman era.