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The problems inherent in the decision to begin simultaneously from opposite sides of the mountain in the hope of meeting head-on in the centre, can be summarised as follows. There is no serviceable landmark visible from both entrances to the tunnel. The shape of the mountain does not permit either entrance to be seen from the summit. There were no clocks to verify that a reading of the angle of the sun’s shadow—nature’s most reliable measurement—would be taken simultaneously in two different sides of the hill. There were no compasses or theodolites. And it should be remembered that any measure of direction or level which was made by a series of smaller, repeated measurements over or around the surface of the hill, was always subject to a compounding margin of error with every successive measurement taken, especially over such rough terrain. An error of one degree between the alignments of the trajectories of the two halves of the tunnel would result in a divergence of about 9m at the point where the two tunnels were to meet.
Given all this, Eupalinos had to ensure three certain ties in his construction: (1) that the angles of the trajectory were the same and perfectly aligned so as to guarantee meeting in the centre; (2) that the two entrances on the opposite sides of the mountain were perforated at exactly the same height above sea level; (3) that the digging maintained both a constant horizontal level and a straight axial line without deviation. An error in any of these would mean failure. Lastly (4), there were purely practical problems, such as the possibility of encounter dangerously unstable rock, or seams of water in the heart of the mountain, which could respectively block or flood the tunnel
Samos Island is part of the Northern Aegean Island Group, Greece.