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.
(Theme music)
