Bathymetry is the measurement of ocean depths, while a test of ocean depth is called a sounding. How were earlier soundings made?
Historical depth soundings were made with weights attached to ropes. To make a sounding, a person (sometimes called a linesman or a leadsman) would drop the weight into the water, releasing about 6 feet of rope at a time. This measurement was called a fathom, which originated as the length between a large man's middle finger tips with arms fully outstretched. To make a measurement, lines were marked with each fathom and dropped into the sea. After the weight hit bottom, the first marking over the water's surface was referenced to obtain the depth of that location. This technique was used by the ancient Greeks and Romans for ocean navigation!
An image from "Description of the Northern Nations" by Olaus Magnus in 1555depicting the use of weight and line for seafloor soundings. Image in the public domain and via Wikimedia Commons.
An image from the "History of the French Navy Illustrated" by Charles de la Ronciere. The image was painted by Antoine Léon Morel-Fatio in 1844 and shows the long-continued use of weight and line for seafloor soundings. Image in the public domain and via Wikimedia Commons.
By the 19th century, Europe and its growing naval operations called for better technology. The British Royal Navy . Edward Massey, a clockmaker, made the first widely-used depth-tracking machine in 1802. When line was released, it turned dials that displayed the approximate depth reached. By 1811, the Royal Navy had purchased 1,750 of them--one for every ship in the Napoleonic wars! This was an immensely important invention in its time. It was limited, however, to shallower depths.
Edward Massey's sounding machine, image in the public domain and via Wikimedia Commons.
In 1876, William Thompson--a very successful scientist also known as Lord Kelvin--invented a weight and line device that could be used for deep-ocean soundings. It was made with piano wire and a much heavier lead weight than used in previous systems. Later versions of his device were motorized and freed sailors from releasing and pulling the rope by hand!
For a couple reasons, line and weight soundings can be found shallow (to make a pun on their depth). Ocean currents could shift the rope and throw off depth measurements; to provide an accurate reading, the weight has to drop straight to the seafloor. Further, rope soundings could only make one measurement at a time, and only with much effort and waiting.
Today, modern soundings are performed with advanced technology. A depth finder or echo sounder is one such tool, and charting underwater depths and structures with these is part of the job description of a hydrographer. An echo sounder sends sonic pulses into the water, records the echoes that comes back, interprets the data, and then creates a map of the seafloor. Because the technology is based on sound waves, it classifies as a kind of sonar. These devices are present on virtually every naval and merchant ship, and also in small vessels. They have served many uses besides seafloor mapping, such as finding fish, protecting ships from shoal water, detecting submarines, and measuring ice thickness in Arctic regions. Today, echo sounders work in fast repetition, making thousands of soundings in a single hour.
The earlier kind of echo sounder is single beam sonar, and while they're still widely used today, their 3D depth profiles are limited to a smaller width than other technologies, since they only send and receive vertical pulses. This technology was invented by Herbert Grove Dorsey around 1930 as the "Dorsey Fathometer".
The more modern multibeam sonar sends multiple pulses from the boat and receives multiple return pulses at once, meaning it can create 3D representations of entire regions of seafloor it is utilized on.
Comparison of the amount of information gathered through popular sounding methods, graphic by NOAA in the public domain.
Today, even satellites aid the quest to map the ocean floor. They can make reliable readings in shallow-water areas where sounding vessels cannot. They can even find undersea mountains by detecting the small changes in surface water elevation that the submerged mountains produce! The data that satellites provide, in conjunction with data obtained from sailing vessels, is what made this bathymetric map possible (shown below).
Photos in the public domain, via NOAA. Credit to Dr. Walter H. F. Smith, NOAA/NESDIS/STAR
The darker the blue, the deeper the ocean is at that location, and vice versa.
Sources:
A timeline of tech development
Satellite bathymetry
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