If you were a sailor, how would you find out if they were any dangers on the sea floor? How would you learn how deep the water is, or how water depth changes from place to place? Before the 20^th century, the most common way to find the depth of the ocean was to use a sounding line. This was a rope with a lead weight attached to the end. The rope was tied in a knot every fathom (6 feet). A sailor would swing the line overhead and off the ship, and count the number of knots that went by before the weight hit the sea floor. This gave a good estimate of depth.

This depth information was very important. It allowed ships to navigate safely through unknown waters without being wrecked. By collecting many observations, early oceanographers learned that the sea floor gradually became deeper offshore. This region was called the continental shelf. However, the longest sounding lines were at most 1600 meters (1000 feet) long. Scientists and sailors quickly discovered that many parts of the ocean were deeper than this, but they could not tell what was there. Most people assumed the deep ocean floor was a broad, flat plain between the continents. 

Early exploration of the ocean bottom.