The Eupalinos aqueduct was built in the 6th century in Greece and was the first tunnel to be built by excavating from both ends through geometric calculation. The completed tunnel stretches just over 1000 meters in length at 1,036 m and served as an aqueduct for the town of Samos, Greece.
The man for which the tunnel gets its namesake, Eupalinos, was an ancient engineer who was commissioned under the tyrant Polycrates to build the tunnel to supply Samos with fresh water. Part of the goal was to provide an underground channel that would otherwise be undetectable to enemies as the source of Samos' fresh water from mountain streams. Through careful geometry and mathematics, Eupalinos and his teams were able to accomplish just that.
[Image Source: Wikimedia]
The finished aqueduct was used for many millennia until it was eventually lost in the middle ages. In the late 1800s, the tunnel system was rediscovered and much of the evidence of its engineering was thus brought to light.
The tunnel collected water from a spring that was on the other side of a mountain in relation to Samos. Teams first covered the spring to conceal it from enemies and began carefully planning construction of the tunnel. Previous aqueducts would have been constructed from one end only, but given the urgency of the project and the desired speed, the ambitious goal of constructing the aqueduct from two ends was set in motion.
To direct the spring to the beginning of the aqueduct, crews created a small buried channel along the mountainside near the surface. To get the water from the beginning of the aqueduct to the other side of the mountain, some careful calculation was needed.
[Image Source: Wikimedia]
The south side of the tunnel was constructed slightly larger from the opposite side due to the stabler rock system present at construction. In terms of size, the northern side is only large enough for one person to fit through in places. In terms of the vertical plane, Eupalinos used lines to keep level and make sure that each tunnel started at the exact same elevation. One of the biggest fears in construction was that a slight error in the beginning of the tunnels would result in them never meeting.
For the horizontal plane, each tunnel had a width of 1.8 meters.The expected meeting point was calculated, but Eupalinos knew that he would never be able to be exact with implementing his measurements. He understood that even an error of just 2 meters would result in the tunnels becoming a failure. To prevent this problem from occurring, as soon as the tunnels were in earshot, about 12 meters in the given rock, he made changes in direction for each digging team. The southern team made about a 15-degree turn to the left and the northern team made the same turn to the right. By making this adjustment, it ensured that the tunnels would definitively meet at some point and prevent them from continuing on in parallel.
Similarly, Eupalinos was concerned about the tunnels never meeting in the vertical plane. When the tunnels got close, he made the heights larger to allow for a higher probability of the tunnels meeting. In the end, the actual elevation of the tunnels was only off by a few decimeters. This is one of the greatest engineering feats of the time given the tools available.
If you are interested in exploring this ancient engineering marvel, it is open to the public in the modern town of Pythagorio.