USS Milwaukee made the first echo soundings of the Puerto Rico Trench in 1939. The U.S. Navy cruiser’s measurements identified an exceptionally deep part of the Atlantic, later known as Milwaukee Deep.
While steaming north of Hispaniola and Puerto Rico on February 14, 1939, USS Milwaukee used its echo-sounding equipment to measure the trench. The original reading was reported as 5,041 fathoms, or about 30,246 feet, though later corrections and surveys produced different depth estimates.
The ship was USS Milwaukee (CL-5), an Omaha-class light cruiser named for the Wisconsin city. Its discovery gave the deepest Atlantic feature its common name. Later vessels, including the U.S. Fish and Wildlife ship Theodore N. Gill, obtained closely comparable measurements, helping confirm the existence of the great depth.
The names Milwaukee Deep and Brownson Deep can cause confusion. Milwaukee Deep commonly identifies the deepest point, while Brownson Deep may refer to the surrounding seabed or a broader area of the Puerto Rico Trench. Modern surveys and a 2018 crewed descent have revised the measured depth, but they do not change which ship made the pioneering 1939 echo soundings.