Bathymetry is the science of mapping or charting the bottoms of bodies of water, including both fresh and marine waters. The simplest technique involves lowering a weighted line into the water until it hits bottom. A measurement of the rope gives the depth from the water surface to the solid bottom. By taking measurements at equal intervals across the surface water, a crude picture of the bottom is constructed. If you were mapping the bottom of a pond, you would want to cross the pond many times to create some kind of grid. In other words, more measurements over the entire pond will give a better picture of the contours of the bottom.
Of course scientists are not cruising the oceans in ships with long ropes to map the sea floor. They employ much more sophisticated technology, including sonar, radar and remote sensing satellite imagery, to name only a few. What the different methods have in common is that they all take advantage of waves of energy from the electromagnetic spectrum. Beams bounced off the bottom are received and recorded as distance data, which is then converted to depth information. It gets a little tricky when the beam is not directly under a ship, but rather pointed at an angle!
To really understand bathymetry, let's think of it as the inverse of mapping land features. On land, all measurements of elevation are above sea level. So the highest peak is at the greatest vertical distance from sea level. With bathymetric measurements, the opposite occurs. An underwater mountain, or seamount, would be recorded as the shortest distance from the surface. The greatest distances from the surface are the deep trenches. Nevertheless, the end result is the same: a 3-D image of the bottom.
Why map the ocean floor? Are we planning to build roads across the Atlantic? Well not exactly. But the oceans are the largest single natural resource on Earth. From them we extract food, water, and salt. We mine the bottom for minerals, drill for oil, lay pipes and cables and even tunnel through the bottom to build underwater roads and railways such as in San Francisco Bay and the English Channel. And finally, submersible vehicles navigate the ocean depths, and sometimes, go looking for giant squid.