How did scientists discover deep ocean trenches?

Back in the early-mid 20th century, before the development of sonar, the navy would have crews sail the ocean, and every so often measure the depth by dropping a weight tied to the end of the rope into the water. The rope was scaled, and the person would then record the depth on that ship's chart. After the development of sonar, however, this became a lot easier. Sonar is very accurate and works by measuring the time it takes for a sound wave to leave, bounce off an object, and come back. These sound waves are shot in all directions underwater, the time lengths are entered into a formula, and then an accurate depth is given. Now they can enter these depths into a computer to create a topographic map of the ocean floor; it is pretty sweet.

Hope this helped