The Tunnel of Eupalinos

Digging tunnels is probably among the most toughest engineering projects the ancient people undertook, because it required mastering several sophisticated fields of science including architecture, geodesy, hydraulics, and geology. This makes ancient tunnels fascinating subjects for study.

The Tunnel of Eupalinos, on the Greek island of Samos, is one such example. Built in the 6th century BC, and first described by Herodotus, the Tunnel of Eupalinos was a major engineering milestone because it is one of the earliest known tunnels that was dug from both ends. Using only picks, chisels and hammers, and relying on simple laws of geometry and trigonometry, these ancient engineers accomplished a nearly impossible feat—meeting beneath a hill more than two hundred meters tall and nearly half a kilometer from either end.

Photo: denvilles_duo/Flickr

The tunnel was dug under the direction of the Greek engineer, Eupalinos of Megara, who many consider to be the first hydraulic engineer in history. The purpose of the tunnel was to supply the ancient capital of Samos, today called Pythagoreion, with fresh water. The city of Samos had outgrown the capacity of the wells and cisterns within the city's limits, but the main source of fresh water on the island was on the other side of Mount Kastro from the city. It was decided that an aqueduct be built under the mountain to keep it concealed from enemies, who might destroy it and cutoff water supply to the city.

The tunnel was dug from both ends and met beneath Mount Kastro’s highest point. In order to make sure the two ends met, Eupalinos arranged for the two straight shafts to be dug towards the meeting point at an angle. This way, Eupalinos had only to ensure the shafts were dug at the same level. As long as the two digging teams maintained level, the two shafts would eventually cross each other. Eupalinos was aware that failing to meet would be catastrophic. So he had the shafts dug in a zigzag fashion near the meeting point. Some scholars argue that the tortures zigzag route the tunnel takes through solid rock was not deliberate but the result of a series of errors and corrections. In the final stretches of the digging, the two teams would have been near enough to hear each other through the rock. They would have changed direction as needed, coordinating their digging effort until they met. The sharp turns near the meeting point attest to this theory. The precision was so outstanding that at the meeting point the vertical offset was only 60 centimeters.

The Tunnel of Eupalinos

The most fascinating thing about the Tunnel of Eupalinos, as Tom M. Apostol writes, is—“when the tunnel was dug, the Greeks had no magnetic compass, no surveying instruments, no topographic maps, nor even much
written mathematics at their disposal. Euclid’s Elements, the first major compendium of ancient mathematics, was written some 200 years later.”

The Tunnel of Eupalinos, however, was neither the first nor the last to be excavated from both ends. The Siloam tunnel in eastern Jerusalem, is older than the Tunnel of Samos by at least a hundred years. It too was dug from both ends, but was much shorter, at 533 meters, than the tunnel in Samos, which was slightly over a kilometer long. Another famous example is the considerably longer Channel Tunnel that was completed in 1994. Even with the full power of modern technology, Channel Tunnel didn’t meet exactly in the middle. When the British and French tunneling teams met under the English Channel, the two shafts were misaligned by 3.5 meters. By comparison, Eupalinos used very little mathematics. How he managed to pull off one of the finest engineering achievements of ancient times is a mystery.

The Tunnel of Eupalinos

Sketch of the tunnel’s cross section, adapted from the original by Tom M. Apostol.

The tunnel itself is 1,036 meters long, and about 2 meters wide and 2 meters high. Along the length of the tunnel a separate channel was dug to convey water, which increases in depth over the course of the tunnel, from 4 meters deep at the north end to nearly 9 meters at the southern end. Vertical shafts link this channel to the main tunnel roughly every ten meters. Within the channel, water was transported in an open-top terracotta drain.

The tunnel and aqueduct was in use for a thousand years, until it was abandoned in the 7th century CE. The tunnel was included in UNESCO’s world cultural heritage list in 1992, but it was only a couple of years ago that the Culture Ministry opened portions of the tunnel to the public.

The Tunnel of Eupalinos