Milwaukee Deep, also known as the Milwaukee Depth, is part of the Puerto Rico Trench. Together with the surrounding area, known as Brownson Deep, the Milwaukee Deep forms an elongated depression that constitutes the floor of the trench. As there is no geomorphological distinction between the two, it has been proposed that the use of both names to refer to distinct areas should be reviewed.
The floor of the Puerto Rico Trench constitutes the deepest point in the Atlantic Ocean. During the Five Deeps Expedition, explorer Victor Vescovo achieved the first crewed descent to this location. Media outlets overwhelmingly referred to the area as the Brownson Deep, while the name Milwaukee Deep was used by others. However, likely due to the factors mentioned above, the expedition has not used any particular name to refer to the site of their Atlantic dive. It is named for the USS Milwaukee which was itself named for the city of Milwaukee.
The Puerto Rico Trench has a maximum depth of no more than 8,376 metres (27,480 ft; 4,580 fathoms), as directly measured by Vescovo during his 2018 descent to the Atlantic Ocean’s deepest point. Previously, however, the depth of the Milwaukee Deep (when being used to refer to the Atlantic's deepest point) had been reported by various sources as 8,710 m (28,580 ft; 4,760 fathoms), 8,740 m (28,670 ft; 4,780 fathoms), or 8,750 m (28,710 ft; 4,780 fathoms). It is just 76.0 miles (122.3 km; 66.0 nmi) north of the coast of Puerto Rico at Punto Palmas Altas in Manatí.
The ocean floor feature is named for the USS Milwaukee (CL-5), a U.S. Navy Omaha class cruiser, which discovered the Milwaukee Deep on February 14, 1939 with a reading of 28,680 feet (8,740 m; 4,780 fathoms). On August 19, 1952, the U.S. Fish and Wildlife vessel Theodore N. Gill obtained a reading of 28,560 feet (8,710 m; 4,760 fathoms) at (19°36′N 68°19′W / 19.600°N 68.317°W), virtually identical with the Milwaukee's reading. By then, the existence of deep water to the Atlantic Ocean side of the Caribbean had been known for more than a century.
One of the area's earliest soundings was obtained June 12, 1852 by Lt. S. P. Lee, U.S. Navy brig Dolphin, with a reading of 22,950 feet (7,000 m; 3,825 fathoms) at (26°32′N 60°06′W / 26.533°N 60.100°W).
In 1964, the French submersible Archimède explored the Puerto Rico Trench to a depth of approximately 8,300 m (27,200 ft; 4,500 fathoms) but did not reach its deepest point.
On 21 December 2018 Victor Vescovo made the first crewed descent to the deepest point of the trench in the Deep-Submergence Vehicle DSV Limiting Factor (a Triton 36000/2 model submersible) and measured a depth of 8,376 m (27,480 ft; 4,580 fathoms) by direct CTD pressure measurements. This made the Limiting Factor the deepest diving operational submersible at the time.
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