Today, let's look for the Tower of Babel. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

The Tigris and Euphrates Rivers flow down through Iraq to the Persian Gulf, just North of Kuwait. Ancient Babylon sat on the Euphrates. It was about 50 miles south of Baghdad, which sits on the Tigris. The old nation of Babylonia centered on those two cities and those two rivers.

In the city of Aqar Quf, north of Babylon and West of Baghdad, a huge clay structure rises 150 feet. It's weathered and formless now. But it once formed the base for a great temple in the sky. It was built around 1400 BC. It's one of the best remains of a structure called a ziggurat.

To understand ziggurats, we must understand the soil along the Tigris and Euphrates. Those great watercourses dump silt during flood season. The silt constantly settles. Building on that substrate is like building on quicksand.

Ancient engineers used a very modern technique to solve the problem. They laid down alternating layers -- first brick, then sandy soil laced with reed matting. They formed huge terraces from these layers. The bricks took the vertical load. The tough matting kept the structure from slumping to the side when it got wet. It was an ancient example of what we call building with composite materials.

The Sumerians made the first ziggurats this way in 4000 BC -- before we had writing, or even the wheel. Nebuchadnezzar built the last great ziggurat in Babylon in 600 BC. Archaeologists locate some twenty ziggurats in the region.

The excavation of Babylon is complete and clear. The tower once rose from its center. We read the inscription on an old slab, found near that spot. It says,

I am Nebuchadnezzar, King of Babylon. I have paved the Babel Street with slabs from Shadu for the Procession of the great God Marduk.

Nebuchadnezzar's ziggurat rose 250 feet above Babel street. Of course, that was after the Genesis story of the Tower of Babel. But Babel Street was also the site of older ziggurats, built and torn down. Maybe one of them was the tower in Genesis.

We're told that the Tower of Babel had to fall because it was pure pretension. It was neither art nor worship. But many ancient ziggurats were much more. They represented the origins of reinforced concrete. They reflected the spirit of the great works of civil engineering. And they -- far from collapsing under the weight of pretension -- still stand today.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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Kerisel, J., Down to Earth. Boston: A.A. Balkema, 1987, pp. 7-13.

Stereopticon photo courtesy of Margaret Culbertson

The ruins of Nebuchadnezzar's Palace in 6th-century Babylon