During World War II, new technology was developed that allowed ships to accurately measure water depths as they sailed across the oceans. These were single beam echo-sounders, instruments that could be attached to the hulls of ships. An echo-sounder works by sending out a sound signal, or "ping," into the water. This sound travels through the water until it reaches the seafloor. When it reaches the seafloor, it bounces off and reflects back up to the ship, where the echo-sounder records the reflected signal. So, the name "echo-sounder" actually tells you how it works—it sends out a "sound" (sounder) and listens for the reflected sound, or "echo." This is basically the same thing that happens when you stand in a big empty room and make a noise—the sound bounces off the walls because they are a hard surface, and you hear the echo of the noise that gets reflected back toward you. The time it takes for the signal to reach the seafloor and return to the ship can be used to calculate the water depth.

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