For a long time, telescopes have enabled astronomers to look millions of miles into space. But no one has never seen the inside of our own earth. The deepest hole ever dug is about eight miles--a deep hole by most standards but still four thousand miles short of the earth's center.
To test their theories about what goes on inside the earth, geologists have developed a technique like a medical x-ray. But in the place of x-rays, they use seismic waves produced by actual earthquakes.
Whenever an earthquake occurs, it sends what are called "seismic waves" through the earth.
Most of the seismic waves are too small to feel, but sensitive electronic equipment enables geologists to convert those waves into information about what's under our feet.
That's because seismic waves travel faster through cooler, more rigid rock than they do through hot or molten rock.
By timing how long it takes the waves to get from one part of the earth to another, geologists construct a three-dimensional picture of the inside of the earth.
Studying the earth with seismic waves is called "seismic tomography" and in principle it's not that different from a medical technique for diagnosing cancer, called "computer-aided tomography," or CAT for short.
A CAT scan takes x-rays from all angles of the body to generate a three-dimensional image of what's inside.
Just as a CAT scan uses x-rays to see tumors in the body, seismic tomography can tell us where the molten lava lies underneath a volcano, where oil is trapped between layers of rock, or even where drums of toxic waste are buried beneath an old landfill.