Only in the last few decades, with the use of modern submersibles (small underwater vehicles) and unmanned remote-operated vehicles (ROVs), have scientists begun to compile more precise measurements of ocean depth. Instead of using sounding lines, these vehicles use sonar : They transmit sound waves, or pings, through the water, timing how long it takes for the pings to bounce, or echo, back from the seafloor. That time is all they need to figure out the depth at that location.
Toothed whales (and bats above ground) have been using sound in the same way (a process you may have heard of called echolocation) to familiarize themselves with their surroundings long before we humans ever thought of it!
Here's how sonar works: Suppose it takes eight seconds for a sound wave to travel to the ocean floor and back. Divide that time in half to get the time it takes for a one-way trip to the bottom. Multiply your answer by the speed of sound in water, which is 4,800' (1,477 m) per second. The distance to the ocean floor is 19,200' (5,907 m)!
Try bouncing sound yourself with this experiment! Supplies
Metal pie plate
Wristwatch (one that makes a ticking noise)
Two paper towel tubes
Hold the metal pie plate on edge. Place your wristwatch in the end of one of the paper towel tubes; have a friend hold the tube so that the empty end is close to but not touching the pie plate. Hold the second tube with one end near the pie plate so that it forms a 45 degree angle with the first tube. Listen through the opposite end of the second tube. You should be able to hear the distinct ticking of the watch.
Can you figure out why the sound waves traveling through the first tube change direction and head up the second tube?
Think of each tick of your watch as a ping of sonar. The sound waves travel straight through the paper towel tube your watch is in until they reach the pie plate. There, the waves bounce off the solid metal surface (just as sonar bounces off the ocean floor) and travel back through the second tube to your ear.
Mapping the ocean floor
Nowadays, ships tow echo-sounders called bathyscans that send out as many as 120 sound waves at once! The results of all these echo soundings are calculated, compiled, and translated by computers into detailed images of the ocean floor. Commercial fishermen use echolocation to locate fish, too, so they'll know the best place to set their lines.
Resources and References
Littlefield, Cindy A. Awesome Ocean Science: Investigating the Secrets of the Underwater World. Williamson Books, 2006.