How Do We Measure Ocean Depth?  
Method #1: Make equally spaced marks on a rope. Lower the rope over the side of a ship until the rope touches the bottom. Count the number of marks that go into the water.

Method #2: Send a sound from the ship towards the sea floor. Measure how long it takes to hear the echo.

In deep water, it takes longer for the sound to bounce off the bottom and get back to the ship.

SONAR - SOund Navigation And Ranging

Bathymetry - from Greek bathy, meaning "depth", and metry, meaning measurement

How Deep is "Deep"?

We usually measure ocean depth in hundreds or thousands of meters. To give you a scale, consider San Francisco's Transamerica Building:

  The Transamerica Building is about 250 meters tall.