The Trieste consisted of a float chamber filled with gasoline for buoyancy, with a separate pressure sphere for two people.
The Trieste was over 15 m (50 ft) long and
the ballast tanks 4m (12ft) in diameter. The majority of this was a
series of floats filled with 85,000 liters (22,500 gallons) of
gasoline, and water ballast tanks were included at either end of the
vessel, as well as releasable iron ballast in two conical hoppers along
the bottom, fore and aft of the crew sphere. The crew occupied the 2.16 m (6.5 ft) pressure sphere, attached to the underside of the float and
accessed from the deck of the vessel by a vertical shaft that
penetrated the float and ran down to the sphere hatch.
January 23, 1960, Trieste reached the ocean floor in the Challenger
Deep (the deepest southern part of the Mariana Trench), carrying
Jacques Piccard and Lieutenant Don Walsh, USN. This
was the first and only time a manned vessel has reached the
deepest point in the Earth's oceans. The onboard systems indicated a
depth of 11,521 metres (37,799 ft), although this was later revised to
10,916 metres (35,814 ft), and more accurate measurements made in 1995
have found the Challenger Deep to be slightly shallower, at 10,911 metres (35,797 ft).
To make the record dive, the Trieste was fitted with a new pressure sphere,
manufactured by the Krupp Steel Works of Essen, Germany. It was in
three finely-machined sections (an equatorial ring and two caps). The sphere's walls were 12.7 centimetres
(5.0 in) thick (it was overdesigned to withstand considerably more than
the rated pressure). The sphere weighed 13 metric tons in air and 8 metric tons in water. The float was necessary
because the sphere was so dense - it was not possible to design a sphere
large enough to hold a person, strong enough to withstand the necessary
pressures, and still light enough to be neutrally-buoyant. Gasoline (petrol) was chosen as the float fluid
because it is less dense than water, yet relatively incompressible even
at extreme pressure, thus retaining its buoyant properties.