Rome's Tremendous Tunnel: The Ancient World's Longest Underground Aqueduct

By Matthias Schulz

Part 2: How Did the Romans Accomplish Such a Feat?

This time the empire pursued an even more ambitious goal. It aimed to place the remaining route underground. That dispensed with the need for bridges. Below the surface, laborers could simply chisel the floor of the tunnel out of rock.

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? After only a few meters, workers would have had trouble breathing in the dusty passageways.

And there were other challenges: With an average height of 2.5 meters (8 feet) and a width of 1.5 meters, only four legionnaires working underground could ensure the advance of the tunnel. They couldn't manage more than 10 centimeters (4 inches) a day. At that rate, they would still be tunneling toward Gadara today.

Surveyors, water engineers and mining experts traveled farther east to find solutions to these problems. Döring has largely deciphered their working methods. "There are many indications that the engineers first traced the surface route and then sank sloping shafts into the rock every 20 to 200 meters," he says. These shafts provided fresh air. What's more, they meant that hundreds of men could work simultaneously on the endeavor.

Dead Chickens

When Emperor Hadrian visited the Decapolis in the 129 B.C., the project was in full swing. To the sound of trumpets the legionnaires and local workers lined up and climbed underground. They labored with pointed chisels in the glow of oil lamps. Slaves hauled the excavated material up the shafts.

Nowadays, the old service entrances make it possible to determine the course taken by the underground water labyrinth. "Nearly all entrances were walled up in ancient times to prevent animals from falling in," explains Döring, "and we found others buried or filled with meters of rubbish." Dead chickens lay in one hole.

How the aqueduct worked.
DER SPIEGEL

How the aqueduct worked.

Like mountain climbers, with one hand on a safety rope, the professor and his helmeted students make their way down the steep steps, which descend at 50-degree angle. With each step it becomes more slippery.

Down below, on the floor of the tunnel, the researchers are surrounded by damp darkness. At times it is so suffocating that the gas monitoring devices begin to peep. Rubble occasionally blocks the passage, creating hip-high ponds of mud and rainwater. In other places, the wind whistles and blows like in a wind tunnel.

The group has unearthed over 300 entranceways. But there remain many unanswered questions. "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.

Down to the Last Centimeter

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.

In view of such difficulties, it's hardly surprising that mistakes were made. Once in a while, the chiseling crews would hammer right past each other. In such cases, the only way to connect the sections was to send tapping signals through the rock and zigzag until the workers met up.

It took 120 years to complete the subterranean enterprise. Then the water finally gushed and bubbled from below. Mineral deposits in one section near Abila reveal that 300 to 700 liters per second rushed through the canal. The genius of Rome had managed to transform this part of the Levant into a veritable Garden of Eden.

And yet there was an overwhelming sense of disappointment in Gadara. Even the mega-aqueduct in Jordan attests to the tragic truth that nothing created by the hand of man is ever perfect. The original plan called for the water to fill a high stone reservoir that would feed the city's fountains and the planned temple to the nymphs.

But that never happened. Since the surveyors ended up making a number of miscalculations, the water -- after over 170 kilometers -- arrived in Gadara slightly too low for the grand plans.

The reservoir could not be filled -- and the fountains never went into operation.

Translated from the German by Paul Cohen

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