The depths of the ocean have been charted since the early days of sailing using a method called sounding. A sounding line (a rope that has a weight attached) is lowered over the side of the ship. When the weight hits the seafloor, the line goes slack, and is marked at the water's surface. The weight is pulled back up and the distance from the surface mark to the weight is measured. This length equals the depth of the ocean at that point. This method of seafloor mapping is very time consuming, especially when charting deep water.
The invention of sonar changed the way that the seafloor is mapped. A combined transmitter and receiver, called a transducer, sends a sound pulse straight down into the water. The pulse moves down through the water and bounces off the ocean bottom. The transducer is able to pick up the reflected sound. Computers precisely measure the time it takes for the sound pulse to reach the bottom and return. In shallow water the sound waves will return very fast and in deeper water it will take more time to receive the echoes. The depth of the ocean is calculated by knowing how fast sound travels in the water (approximately 1,500 meters per second). This method of seafloor mapping is called echosounding.
Echosounders can use different frequencies of sound to find out different things about the ocean. Water depth is typically measured by echosounders that transmit sound at 12 kiloHertz (kHz). Lower frequencies (3.5 kHz) can be used to look at the layers of sediments below the seafloor (See How is sound used to study the Earth's history?). Higher frequencies (200 kHz) can be used to identify fish and plankton that are in the water column (See Fishing).