Today, we build a cathedral and pay the piper for it. 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.

We read of the great constructions of ancient Egypt -- the Pyramids, Karnak, endless burial monuments -- each one grander than the one before it. All that stone! The Great Pyramid alone is made from forty million cubic feet of rock.

Now here's a startling statistic. The limestone quarries of Northern France produced more stone during 300 years in the High Middle Ages than was used in all of ancient Egypt. Those magnificent Gothic cathedrals and other late medieval buildings ate an enormous amount of rock. Some of the cathedrals were almost as tall as the Great Pyramid, and so many were built! So much stone!

Furthermore, the blood of the Battle of Hastings was hardly dry when William the Conqueror exported stone to England to build a Battle Chapel. After that we find huge quantities of rock moving across the Channel: stone for Canterbury Cathedral, stone for Westminster Abbey, stone for Winchester Castle, stone for Norwich Cathedral ...

Some of this rock came from open pits and some from underground mines. 800 years later, dirt has filled in most of the old open pits, and grass has grown over them. The underground stone quarries are a different matter. They remain. Some have been sealed off. Some are now used to grow mushrooms. Some, like the one at Saint-Leu d'Esserent, north of Paris, are still in use. It reaches so far into a hill that stone has to be hauled through a mile of tunnel. Here and there its caverns open into workshop areas where stone is shaped before it's taken out. The Germans used these areas to manufacture rocket engines, safe from allied bombs, during WW-II.

Paris itself sits on limestone, and the earth beneath it is a rabbit-warren of tunnels. The famed Paris Metro has 190 kilometers of underground lines, but the quarries crisscrossing under the city run twice that distance. Parisians keep a close eye on those caverns -- not out of archaeological or historical interest, but out of an ongoing concern for the structural safety of the city above.

These tunnels remind us that our works have been altering the face of the earth for a long time. The flight of imagination that propelled the magnificent Gothic cathedrals into the sky was rooted deep in the earth. Many of those glorious buildings remain, but so, too, do the empty cocoons from which they were taken. They remind us of a natural balance of payments that we can never escape when we build things.

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

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Gimpel, J., The Medieval Machine, The Industrial Revolution of the Middle Ages. New York: Penguin Books, 1976, Chapter 3.