A 2500-Year-Old Tunnel Dug From Both Sides of a Mountain Remains a Mystery

About 2,500 years ago, two tunnels dug from opposite sides of a mountain met in the darkness over a kilometer away. This is the Eupalinos tunnel on the Greek island of Samos, with a total length of 1,036 meters. While digging from both sides simultaneously can speed up construction, if the direction deviates by just 1 degree while one side digs 500 meters, the ends can be off by about 9 meters. Researchers believe they precisely set the direction and gradient of the two entrances and continuously re-measured and corrected the course during construction. In reality, the northern tunnel deviated from a straight line, corrected its direction multiple times, and eventually curved back towards the southern tunnel. Furthermore, the main passage for people was almost flat, so they dug a separate deep trench next to the passage, lowered the floor further towards the south, and laid pipes inside to drain water.