How do submarine pilots know where they are going when they are underwater?

Submarine pilots use a periscope to navigate their direction if they are not too far below the water’s surface. This tall, rotating, tube-shaped instrument can be raised above the water’s surface to view surroundings, using a series of mirrors and lenses inside to relay images. Beyond that, submarines use sonar (sound waves) to make echo soundings of their surroundings. Transmitted sound waves are reflected off objects or the ocean floor; the time it takes for these sound waves to be reflected back indicates how far away things are located. The echoes are then converted into electrical signals that appear on a display screen, which gives a picture—similar to that of an airport’s radar screen—of surrounding waters.

Submarines fill or empty their ballast tanks with water in order to float or sink under water.