Short answers, pulled from the story.
Jacques Piccard and United States Navy Lieutenant Donald Walsh first reached the bottom of the Challenger Deep in the Mariana Trench in 1960, descending to a depth of 10,915 m in the bathyscaphe Trieste. They observed fish and other deep-sea organisms at that depth.
The HMS Challenger expedition, conducted from 1872 to 1876, discovered more than 4,700 new species of marine life and provided the first systematic view of major seafloor features including deep ocean basins. The expedition covered 127,653 km under the direction of Charles Wyville Thomson.
Remote operated vehicles, or ROVs, used in deep-sea exploration can reach depths of up to 6,000 m. They are connected to a surface ship by cable and piloted in real time by operators above.
Dr. Kathryn Sullivan and Vanessa O'Brien became the first women to reach the bottom of Challenger Deep in 2020, descending to 10,925 m as mission specialists aboard the vessel Limiting Factor, piloted by Victor Vescovo.
The Abyssus theory was Edward Forbes's 1843 claim that life cannot exist in ocean waters deeper than 550 m. Michael Sars refuted it in 1850 by finding rich deep-sea fauna at 800 m near Lofoten. The theory was further discredited in 1864 when Michael Sars and Georg Ossian Sars retrieved a living stalked crinoid from 3,109 m.
The most commonly used metals for deep-sea submersible pressure vessels are wrought alloys of aluminum, steel, and titanium. Aluminum suits medium-depth work, steel offers very high yield strength but is heavy, and titanium is nearly as strong as steel while being about three times lighter. The Deepsea Challenger used a steel pilot sphere estimated to withstand 23,100 psi.