World war II,
German U-boats were on the prowl.
To track them the allied forces developed new sonar methods,
And scientists were enlisted to help survey the ocean floor.
When the United States entered the war Harry Hess was a geology professor at Princeton University,
but he also happened to be a navy reservist.
So it wasn't long before he found himself in command of an
attack transport ship in the Pacific.
to help maneuver when coming in for a beach landing
Hess's ship was equipped with a depth sounder,
and still being a geologist at heart 
he used the sounder to measure the depth of the ocean floor whenever a ship was out to sea.
Now what he discovered startled him.
Until the Second World War, most scientist imagined the bottom of the ocean looked like this:
Flat, wide, with nothing but sediment.
But two miles beneath the waves of the Pacific ocean
Harry Hess discovered something else entirely.
Mountains like these here in California with deep canyons and trenches.
Hundreds of high peaks that we now believe were once active volcanoes.
And all of this at the bottom of the Pacific ocean.
Surprisingly though, the discovery of the Pacific mountain range is not what makes
Harry Hess part of your great 100.
Now, we'll get to that in a minute.
To understand where all of this is headed,
I'd like to skip ahead to another event that set the geology world buzzing.
For years oceanographers surveying the Atlantic ocean had taken sonar readings that indicated there was something down there, something big.
In 1953 they found out what it was.
A 12,000 mile long mountain range.
They called it the mid-Atlantic ridge.
The reason it's so great --
To fill us in I paid a visit to Neil Driscoll
a geologist at the Scripts Institution of Oceanography.
Neil:  One of the big discoveries that was made,
was that there was this ridge of underwater volcanoes that stood high above the sea floor.
Bill:  How high is a mountain in the middle of the Atlantic?
Neil:  The average sea floor depths are on the order of about four to five thousand meters.
The mid-ocean ridge sits up at about 2,500 meters.
So they sit about 2 1/2 kilometers on average higher than the surrounding sea floor that's shown here in the deep blue colors.
Bill:  So that's over a mile high?
Neil:  Yes. 
 Bill:  And that's where Harry Hess comes back into the story.
Analyzing core samples and sonar readings from around the mid-Atlantic ridge
Hess made an astonishing discovery.
A phenomenon almost beyond comprehension. 
The age of the Atlantic ocean floor, he determines, was progressively older the further it moved away from the ridge.
Harry Hess had discovered that the sea floor was spreading.
He concluded that molten rock was being forced up from inside the earth at the ridge.
Where it then formed into new crust on the ocean floor.
Gradually it was pushed away on either side as more molten rock continued pushing from behind it.
Hess called his great discovery, "Sea Floor Spreading."
Neil:  Harry Hess was in the position that he could bring it all together,
things were spreading apart and new earth was being generated,
but if you did this for long enough the earth should grow.  And it doesn't.
Bill:  The earth doesn't get any bigger?
Neil:  No.  Harry appreciated the fact that if new earth was being generated in one area
they have to be consumed or recycled in another area.
Bill:  The process that recycles the crust of the spreading ocean floor back inside the earth is called subduction,
but as our next great discovery revealed it's all part of a much larger process.
Perhaps the most powerful force on the face of the earth. 
