I was reading about Sir Richard Branson's new Challenger Submersible and was reminded of the 1960 descent into the Marianas Trench by Jacques Piccard and Don Walsh. I remember reading about the mission when I was kid, in one of my "How? Why? Wonder" books but looking at it now it's just flat out amazing.

Bathyscaphe Trieste

How did bathyscaphe Trieste come to make the deepest dive?

It was the late 1950s and the Cold War was escalating to new levels. As the age of the nuclear submarine dawned, the strategic importance of the undersea world was beyond dispute. But the vast majority of the ocean – the deep sea – was completely unknown and inaccessible.

In 1959 the US Navy responded with an ambitious new program. Its objective was to test whether a new class of extreme-depth submersible – the bathyscaphe – could accomplish scientific and military tasks in the abyss. The program was codenamed Project Nekton. For its setting the Navy chose the deepest place in the ocean, the Marianas Trench, off the American territory of Guam.

At the heart of Project Nekton was the bathyscaphe Trieste. Brainchild of Auguste Piccard, it possessed unparalleled deep-diving abilities – at least in theory. The Navy would test the strange vehicle to its limits. Project Nekton’s crowning glory would be a record-setting dive to the Challenger Deep, the deepest hole in the Marianas Trench, 35,800 feet deep.

Bathyscaphe Trieste
Piccard's first bathyscaph, the FNRS-2, was referred to as the "submarine balloon" because its heavy-metal ballast, attached by electromagnets, allowed it to sink to a desired depth when engaged and rise to the surface when released. It had greater maneuverability than the Bathysphere, though it did not well in tests. Piccard and his son Jacques later designed and built a new bathyscaph, the Trieste.
The bathyscaphe ("bathy" = deep, "scaphe" = ship) Trieste was built in 1953 at Naples, Italy, by the Swiss scientist Auguste Piccard and his son Jacques . The Trieste, essentially a steel sphere attached to large float filled with aviation petrol for buoyancy, lacked manipulators, or samplers, and it was quite large and not very maneuverable. But it allowed unprecedented observations from the water surface down to the benthos (organisms that live on or in the ocean bottom) and provided a tantalizing glimpse of future discoveries.
After several years of operations in the Mediterranean, she was purchased by the US Navy in 1958, under the sponsorship of Office of Naval Research. Transported to San DiegoCA she conducted tests in the Pacific during the next several years. In October 1959, after being fitted with a stronger pressure sphere, Triestewas transported to the mid-Pacific to participate in Project "Nekton", in which she conducted a series of very deep dives in the Marianas Trench. On 27 January 1960 the Triestetook two men, Lt. Don Walsh US Navy and Jacques Piccard, to a depth of 35,800ft (10,910 meters), the deepest spot in the ocean, in the Mariana Trench near Guam. It took 5 hours to fall 7 miles, and when the explorers reached the bottom they stayed 20 minutes. No one has been able to come near that depth since then. At this depth, the pressure is over 8 tons per square inch (some 1,000atm). In 1963, she went to the Atlantic to search for the lost submarine USS Thresher (SSN-593). In August 1963, Trieste found Thresher's remains off New England, 1400 fathoms below the surface. The bathyscaphe was retired soon after that, and some of her components were used in the newly constructed Trieste II (DSV-1).
Trieste is currently on display at the Washington Navy Yard museum.
Click Bathysphere, FNRS-2 or Auguste Piccard for more historical information.
The Trieste II, DSV-1, submersible
In March 1965, with a new float assembly installed the redesignated Trieste II submersible was developed. The new mission specified (in its second form) was testing and evaluating deep submergence concepts, equipment and sensors. After many successful dives, Trieste II submersible was taken into dry-dock and overhauled into yet another configuration. Designated the Trieste II (DSV-1) it was certified to dive to 12,500 feet. The Trieste II has seen many changes and modifications since its overhaul in 1970. Different scientific equipment was added and removed with each individual dive depending the dive mission. Throughout its transforming life, The Trieste submersible was one the most significant deep submergence submersible vehicles ever created both in the eyes of undersea technology and research.
Technical specifications, Trieste-II:

Displacement (srf/sub tons): un known/305
Dimensions (L*B*D feet): 78'0*16'0*21'0
Propulsion:battery operatedelectric motors, 1*main motor with one propeller, 2*external motors
Speed (srf/sub knots):
un known
Range (sub n/miles@knots): 24@2
Diving depth (feet): 20,000 (6,300m)
Complement: 3 crew members
Trieste II is currently on display at the Keyport Submarine Museum, Washington, USA