Roman engineers spent a century digging an underground aqueduct through 66 miles of stone in order to bring water to Syria. Before this was discovered, the longest known underground water aqueduct was in Bologna; it was a mere 12 miles long.
The soldiers chiseled over 600,000 cubic meters of stone from the ground -- or the equivalent of one-quarter of the Great Pyramid of Cheops.Much more information and more pictures at the Der Spiegel link. (click photos to enlarge)
"Over the first 60 kilometers, the tunnel has a gradient of 0.3 per thousand," explains the project director. That works out to 30 centimeters per kilometer -- an astonishingly shallow angle of descent...
But the project faced daunting hurdles. The compass was unknown in the ancient world. How were they to orient themselves within the mountain? And how to provide adequate ventilation inside the tunnels?
The Romans did have levels, a six-meter long design called a chorobate copied from the Persians. They also filled goat intestines with water to find a level around corners. But that alone does not explain the precision of this amazing aqueduct.
"First the surveyors had to establish a uniform route with posts that extended for many kilometers," Döring points out. That alone was extremely difficult on the uneven terrain. Then they had to transfer this line deep below the surface and determine the location of the tunnel floor down to the very last centimeter. But how did they accomplish this with such a high degree of precision? It wasn't possible to lower a plumb line because the construction shafts descended at an angle.