When the earliest mariners sailed into the deep blue sea, they had no idea just how deep it was. The bottom might have been just out of sight, or it might have been limitless. But human curiosity being what it is, they had to find out. And they’re still doing it today, only with more sophisticated techniques.
The first technique was the most obvious -- drop a weighted line over the side of a ship and measure how far the weight falls. It was first used more than two millennia ago, and found that the middle of the Mediterranean was more than a mile deep.
In the early 20th century, scientists turned to a new technology: sonar. A beam of sound is aimed straight down from a surface vessel. From the length of time it takes the sound waves to hit the bottom and bounce back to the surface, scientists can determine the depth.
Technology has provided a couple of newer depth-measuring techniques as well.
One beams a laser from a low-flying airplane to the seafloor. The technique provides accurate measurements, but its depth is limited to a maximum of only a couple of hundred feet.
Orbiting satellites provide global measurements. The contours of the ocean floor have an effect on sea-surface elevation. So precise elevation maps of the surface can reveal a rough contour of the ocean floor.
Using all of these tools, scientists have charted the depth of most of the ocean floor. And they’ve found the deepest known spot on the planet: the Challenger Deep, with a maximum depth of almost seven miles.
copyright 2006, The University of Texas Marine Science Institute