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.
"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.
When they were not too busy conquering distant lands, the Romans liked to dig. German hydromechanics professor Mathias Döring discovered that Roman engineers spent a century digging a 66-miles long underground aqueduct to bring water to modern day Syria: