Samos, or Eupalinus Tunnel is located on the Greek island of Samos. It was built in the 6th century BC by the Greek engineer Eupalinus by order of Polycrates, a cruel pirate who forcibly seized power on the island.
Later, Herodotus ranked this tunnel among the "seven wonders of the world" and gave it a detailed description. By the way, Samos was one of the centers of Greek culture, and of the mentioned "seven wonders of the world" there were two more - the temple of the goddess Hera and the dam in the port (which, unfortunately, has not survived). Many figures of ancient Greek culture were born on the same island.
The Samos tunnel was a unique structure for its time. It served as an aqueduct and has been used in this function for thousands of years. Then it was abandoned, but at the end of the nineteenth century it was reopened.
Having come to power and having committed a series of robberies (and he robbed everyone indiscriminately - both enemies and friends), using the riches obtained from this activity, Polycrates launched large-scale construction on the island, and one of his projects was this very tunnel. The underground location of the aqueduct made sense: if the island was besieged by enemies (which happened very often in those days), they would not be able to block this water conduit and the population would not die of thirst.
The uniqueness of this structure is also manifested in the fact that it was built by laying from both sides at once. When the workers met, it turned out that the discrepancy was relatively small - 10 meters in the horizontal direction and three meters in the vertical direction. To evaluate this result, it is enough for comparison to cite a similar aqueduct in Jerusalem, built much later - in the 8th century AD. Then the correctness of the work was continuously monitored with the help of vertical wells, but the tunnel still turned out in the form of a zigzag and was much longer than intended. It is known that even in the 20th century, the accuracy of tunneling was not always higher than during the construction of the Samos water conduit.
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In general, the construction of tunnels is a very difficult job, especially since it is very difficult to control the process. Laying becomes even more difficult if the mentioned method is used (digging from both sides). It is amazing how the earliest Samos builders achieved such high precision, without modern tools and methods of work. However, Eupalin did use some tricks. It is known that he had a sight - a simple device for horizontal measurement. It is also assumed that the architect could have used a full-scale model of the future structure, located in an open place.
The ancient architect took the construction of the aqueduct very seriously. He used a geometric approach, and many of his calculations were "officially" introduced into science much later, when they were formulated by Euclid. Eupalin realized that it would be almost impossible to control the construction of the tunnel, so he used some engineering tricks that ensured that the builders would meet each other, even if initially their paths were parallel and far from each other. According to the architect's plan, at a certain moment, both groups of builders had to change the direction of the laying in order to certainly get into the opposite half of the tunnel.
The deep antiquity of the structure and its high (in a relative sense) quality, the Samos aqueduct can be considered a true masterpiece of ancient architecture.