The tunnel is inscribed on the UNESCO World Heritage List along with the nearby Pythagoreion and Heraion of Samos, and it was designated as an International Historic Civil Engineering Landmark in 2017.

[3][4] The Eupalinian aqueduct is described by Herodotus (Histories 3.60): The tunnel might also be referred to in the Homeric Hymn to Apollo, which mentions "watered Samos.

"[6] The tunnel was dug in the mid-6th century BC, by two groups working under the direction of the engineer Eupalinos from Megara, in order to supply the ancient capital of Samos (today called Pythagoreion) with fresh water.

It was of the utmost defensive importance; because the aqueduct ran underground, it could not easily be found by an enemy, who might otherwise cut off the water supply.

Aideen Carty suggests that it should be connected with the regime that overthrew the Geomori in the early sixth century BC, which granted citizenship to a large number of Megarians, perhaps including Eupalinos.

[8]: 16 & 35 The tunnel took water from an inland spring, located about 52 metres (171 ft) above sea level near the modern village of Ayiades.

Two rectangular openings, each measuring 28 by 26 centimetres (11 by 10 in), feed the water into a large reservoir with a roughly elliptical ground plan.

At some point before the nineteenth century, a church dedicated to St John was built over the top of this reservoir, further hiding the spring's location.

Vertical shafts were dug from the surface at intervals of 30–50 metres (98–164 ft) and then linked up to create a short tunnel, which brings the water.

In the Roman Imperial period, barrel vaults were built with small stones and plaster to reinforce other sections of the tunnel.

Apparently, the subsidence at the spring lowered the level of the water after work had begun, leaving the tunnel too high.

On the west wall, there are letters in alphabetical order at a regular interval of 20.59 metres (67.6 ft), which indicate that this was the basic unit of measurement used by Eupalinos (it is one fiftieth of the planned course through the mountain).

[8]: 31–35 In the seventh century AD, when the aqueduct had ceased to operate, the southern section of the tunnel was converted to serve as refuge.

This included the construction of a cistern 400 metres (1,300 ft) from the southern entrance to collect water dripping from a vein in the rock.

This is shown by a point in the southern half of the tunnel where the course accidentally diverged to the west and had to be corrected; a notch has been cut out of the rock on the inside of the curve, in order to restore the sight line.

[8]: 41–42 After 273 metres (896 ft) from the northern end, an area full of water, weak rock and mud forced Eupalinos to modify his plan and direct the tunnel to the west.

[9][8]: 52–55 The north and south halves of the tunnel meet in the middle of the mountain at a dog-leg, a technique to assure they did not miss each other (This method is documented by Hermann J. Kienast and other researchers).

With a length of 1,036 metres (3,399 ft), the Eupalinian subterranean aqueduct is famous today as one of the masterpieces of ancient engineering.

This gave a catching width that was wider by 17 metres (56 ft), so that a crossing point would be guaranteed, even if the tunnels were previously parallel and far away.

This proved too difficult and the effort was called off, but it allowed Ernst Fabricius to investigate the tunnel on behalf of the German Archaeological Institute.