Today, we map the ocean floor. 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.

What's at the bottom of the sea? That question has nagged us through most of human history. The very word unfathomable means something that's as far out of reach as the ocean bottom.

The Greeks thought the underwater landscape should mirror the land above. That was better thinking than much of what followed. When Magellan failed to find the bottom of the Pacific with a half-mile sounding line in 1542, he pronounced the ocean immeasurable -- whatever that meant. 200 years later, natural philosophers were estimating ocean depths of over 30 miles.

Ben Franklin's great-grandson Alexander Bache made an ingenious estimate in 1856. He studied records of a tidal wave that'd taken 12 hours to cross the Pacific. He knew how wave speeds depend on depth, so he calculated a 2 1/5-mile average depth for the Pacific. He came within 15 percent of the right value, but knowing an average depth is a far cry from knowing the real shape of the ocean bottom.

Sixteen years earlier, the first deep ocean sounding was made by the Antarctic explorer James Ross. He dropped a 75-pound sinker on a very long line. Once the sinker hit bottom, the line itself was still very heavy. Ross tried to solve that by timing each 600 foot length of line as it fell. When the line slowed slightly, he knew the sinker had hit bottom. That was pretty crude, and the reading was off by a third of a mile.

Besides, one data point doesn't tell much about a very large ocean. Throughout the 19th century, sounding methods were made systematic. By 1876, a depth of over six miles had been recorded in the Pacific -- just over the height of Everest.

Acoustic echo sounding was a 20th-century invention. It made the whole business a lot easier, but it was still done point by point. Continuous sounding didn't come into use until after WW-II. When it did, a great panorama opened up. Before 1850, people guessed the ocean floor was smooth and featureless. Now maps display great mountains and canyons. The Greeks had been right all along. The deepest ocean trench isn't much farther below sea level than the highest mountain is above it.

And as the features unfold, so too do the great mysteries of earth's formation. Only when the mosaic of subsurface detail fell into place could we learn how plate tectonics slipped and slid about earth's surface to form its continents.

A modern survey-ship captain, new to the game, watches his sonar sketch a mountain, miles below. He flinches nervously. Is the bottom rising to scrape the hull? He knows it can't, but he fears it will. There's still mystery in the water under him.

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

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Wilford, J.N., A Continent Beneath the Ice. The Mapmakers. New York, Vintage Books, Random House, 1982, Chapter 19.