The Tunnel of Eupalinos or Eupalinian aqueduct (Modern Greek: Efpalinio orygma, Ευπαλίνειο όρυγμα) is a tunnel of 1,036 m (3,399 ft) length in Samos, Greece, built in the sixth century BC to serve as an aqueduct. The tunnel is the second known tunnel in history which was excavated from both ends (Ancient Greek: αμφίστομον, amphistomon, "having two openings"), and the first with a geometry-based approach in doing so. Today it is a popular tourist attraction.
In the sixth century BC, Samos was ruled by the famous tyrant Polycrates.
During his reign, two groups working under the direction of the engineer Eupalinos from Megara dug a tunnel through Mount Kastro to build an aqueduct to supply the ancient capital of Samos, (which today is called Pythagoreion), with fresh water. This was of utmost defensive importance, as the aqueduct ran underground it was not easily found by an enemy who could otherwise cut off the water supply.
The Eupalinian aqueduct was used for a thousand years, as proved from archaeological findings. It was rediscovered in 1882-1884 and today is open to visitors.
The Eupalinian aqueduct or ditch, is cited by Herodotus (Histories 3.60), without whom it would not have been discovered:
And about the Samians I have spoken at greater length, because they have three works which are greater than any others that have been made by Hellenes: first a passage beginning from below and open at both ends, dug through a mountain not less than a hundred and fifty fathoms [200 m] in height; the length of the passage is seven furlongs and the height and breadth each eight feet, and throughout the whole of it another passage has been dug twenty cubits in depth and three feet in breadth, through which the water is conducted and comes by the pipes to the city, brought from an abundant spring: and the designer of this work was a Megarian, Eupalinos the son of Naustrophos. This is one of the three;...The tunnel took water from an inland spring, which was roofed over and thus concealed from enemies. A buried channel, with periodic inspection shafts, winds along the hillside to the northern tunnel mouth. A similar hidden channel, buried just below the surface of the ground, leads from the southern exit eastwards to the town of Pythagoreion.
In the mountain itself, the water used to flow in pipes in a separate channel several metres below the human access channel, connected to it by shafts or by a trench.
The southern half of the tunnel was dug to a larger dimensions than the northern half, which in places is only just wide enough for one person to squeeze through, and has a pointed roof of stone slabs to prevent rockfalls. The southern half, by contrast, benefits from being dug through a stabler rock stratum.
The two headings meet at a dogs-leg, a technique which was used to avoid the two tunnels missing each other, as explained in the paragraph "Surveying techniques".
The method Eupalinos employed to make the two groups meet in the middle of the mountain, is documented by Hermann J. Kienast and other researchers. In planning the digging, Eupalinos used what are now well-known principles of geometry, which were codified by Euclid several centuries later. With a length of 1,036 metres, the Eupalinian subterranean aqueduct is famous today as one of the masterpieces of ancient engineering.
Eupalinos was aware that mistakes in measurement could make him miss the meeting point of the two teams, either horizontally or vertically. He therefore employed the following techniques:
Since two parallel lines never meet, Eupalinos recognized that a mistake of more than two metres horizontally (approximate cross section was 1.8 by 1.8 m), would make him miss the meeting point. Having calculated the expected position of the meeting point, he changed the direction of both tunnels, as shown in the picture (one to the left and the other to the right), so that a crossing point would be guaranteed, even if the tunnels were previously parallel and far away.
Similarly, there was a possibility of deviations in the vertical sense, even though his measurements were quite accurate; Kienast reports a vertical difference in the opening of the tunnels of only four centimetres. However, Eupalinos could not take a chance. He increased the possibility of the two tunnels meeting each other, by increasing the height of both tunnels. In the north tunnel he kept the floor horizontal and increased the height of the roof, while in the south tunnel, he kept the roof horizontal and increased the height by changing the level of the floor. His precautions in the vertical sense proved unnecessary, since measurements show that there was practically no mistake.
