The Tunnel of Eupalinos Revisited

In the April 1991 issue of BENCHMARKS I wrote about the tunnel of Eupalinos on the island of Samos. Recently I have been fortunate to obtain a copy of a paper that was published in Germany in 1984 (* see note). It seems that the Samos tunnel has fired the imagination of the public as well as archaeologists and engineers since it was first discovered in 1881. In 1959 German surveyor Wolfgang Kastenbein triangulated across the hill pierced by the tunnel and from an accurate determination of both tunnel entrances calculated the theoretical tunnel axis. From 1971 to 1974 archaeologists of the German Archaeological Institute (DAI) in Athens excavated and cleared the entire tunnel and connecting aqueduct, and in the following year resurveyed the whole project. The resurvey disclosed that the work of the original surveyor, whether it was Eupalinos personally or somebody he employed, was worse than I described tongue in cheek in my article.

Kastenbein determined the theoretical tunnel axis to be 3,382.5 ft. long, extending NNW to SSE. The difference in elevation between both entrances is only 1.87 ft. I like to remind my readers that the sole purpose of the tunnel was to carry a waterline sloping about 0.5%. Therefore this represents a leveling error of 15 feet, unless the designers of the tunnel were so dumb as having planned it to be level. While a level tunnel may have shortened the construction length by roughly 230 ft. it would have caused an enormous increase in the excavation for the waterline. Since the tunnel entrance was already at least 10 ft. too high, the volume of the material dug out of that portion of the ditch that is inside the tunnel is 7% greater than the entire excavated material coming out of the tunnel itself. Polycrates should have put the thumbscrews on these guys.  

The tunnel pierces Mt. Kastro 540 ft. below its summit. The cross section is almost uniformly 5.9 ft. by 5.9 ft. Measured along the centerline as constructed, the tunnel is 3,450.15 ft. long, or almost 68 ft. longer than planned. These 68 extra feet are the result of deviations from the planned axis, and the meandering attempt to find the meeting point of the north and south adits.       

The first 845 ft. of the north adit and the first 1,318 ft. of the south adit are straight. At the ending points crew "North" was about 35 ft. east and crew "South" about 50 ft. west of the true axis. That fact would not have been known to the diggers, so why was it necessary to change direction? It is possible that the surveyor had noticed a difference of about two degrees in the bearings of the two adits that were still 1,220 ft. from meeting each other, and he was beginning to get a queasy feeling in the stomach. Neither adit was on course and by continuing on their headings they would have missed each other at the midpoint by about 275 ft. Eupalinus was well on his way of digging two tunnels. The discovery probably created enough of a stir to cause the work in the south adit to be halted.

We can only guess at the sequence of the actual events. The fact that the heading in the north adit was changed several times seems to indicate that the surveyor was not certain whether he was to the east or to the west of the axis, but must have known that he was still a long way from making a connection with the south adit.

There now commenced a zigzag digging in the north adit. Already too far east, a small correction was made and for the next 68 ft. the miners dug even further into the wrong direction. After another and more drastic change in bearing, this time to the west, the tunnel crossed the ideal (and to the surveyor unknown) true axis, and 440 ft. further down its course was now about 180 ft. west of where it should have been. Having aimed for zig the surveyor now headed for zag, and in the following 440 ft. crossed the axis for the second time and ended up 100 ft. to the west of it. The last 237 feet were dug in an irregular line that was at first more or less parallel to the axis. At some point along their advance the northern miners may have heard the faint echo of tapping signals from the south adit because suddenly the north adit veers toward it. Digging in the south adit was now resumed, also guided toward the hammering sounds of their opposites, and at last, 101 ft. later they met.

Even a layman walking through the tunnel with a flashlight will notice that it is not straight. While this may look bad, and while for the purpose the tunnel was built it didn’t matter much, together with the elevation error it more than doubled the construction cost.

As mentioned in my 1991 article, admiration of the Samos tunnel dates back to Herodotus’ time (c.484 to c.425 B.C.) and continues today. It has been called “The eights wonder of the world”, a sobriquet that this writer would have liked to attach to more than one survey he has had the misfortune to encounter.

Because of growing public interest in the architecture of ancient Greece, German television in 1978 aired a film "Water for Polycrates". In it DAI asked the public for input to solve a puzzle: Why does the tunnel deviate as much as 100 feet from its axis? A valid question indeed. It led to interesting speculation as to how the tunnel may have been surveyed. Author Peters suggested that the zigzag course was deliberate to assure that the straight advancing south adit would not be missed, but even at the second "zag" the south adit was over 500 ft. too short to make a connection possible. Very puzzling to say the least.

Both, Greek geometry and development of surveying instruments were adequate for the task at hand, needing only a surveyor with sufficient competence to employ them. Would Eupalinos have attempted to tunnel from opposite sides of a mountain had not the surveying skills and methods of his time given him a reasonable assurance that the two adits would meet? Meet they did of course, but only after torturous meandering through solid limestone, hardly an indication of great competence.

I very much admire the engineering achievements of the antique but I have my problem with this Samos tunnel. I would have found me a better surveyor or dug it all from one side, even in 530 B.C.     

* Note: Konrad Peters: Der Tunnel, das Eupalineion auf der Insel Samos. Dortmund, Germany, 1984. In his article, author Peters does not give sufficient data to allow for a precise determination of the deviation of the tunnel centerline from its theoretical axis. My figures are a result of scaling from the included drawings and are therefore approximate.