First bathyscaphe

The first bathyscaphe was the FNRS-2, designed by Swiss scientist-adventurer Auguste Piccard and funded by the Belgian Fonds National de la Recherche Scientifique (whose initials gave the submersible its name). Work started on the FNRS-2 in 1937, but was halted by the outbreak of World War II. It was completed after the war's end, and made its first test dive (to just 25.6 m, or 84 ft) on 26 October 1948.

Piccard regarded the Bathyscaphe – a word he coined by combining the Greek words bathys ("deep") and skaphos ("vessel") – as the underwater counterpart to the high-altitude balloons (including the record-setting FNRS-1) that he had built in the 1930s. The basic design was something Piccard had devised in 1905, when still at university. It combined a section known as the "balloon" or "float" with a "gondola" or crew compartment in the form of a spherical pressure vessel.

A typical submarine holds its crew in a reinforced pressure hull, with buoyancy controlled by pumping air in and out of a series of attached floatation/ballast tanks. This design can only work down to a depth of a few hundred metres, however, before the pressure becomes too great. Non-spherical pressure hulls can't exceed a certain "crush depth" and the system of air-filled tanks becomes impractical when pressures reach a certain level.

To get past these limitations, Piccard designed the bathyscaphe, which used floatation tanks filled with gasoline (or petrol) instead of air. Gasoline is incompressible like water, but has far lower density. This meant that the floatation tanks could be made of lightweight materials but still resist extreme pressures, providing enough bouyancy to support an extremely heavy and dense spherical pressure hull.

To get the vessel to dive, it would be lowered into the water with two on-board hoppers filled with metal shot, like the ballast in a hot-air balloon. This ballast would cause it to sink, but could be released to control the rate of descent and then further releases would to control the rate of ascent.

The FNRS-2 was built to be deployed from a Belgian research ship called the Scaldis. In the autumn of 1948, the Scaldis sailed to a point off the coast of Senegal to conduct sea trials. It was assisted by a French navy research ship called the Élie Monnier, commanded by Captain Jacques-Yves Cousteau. The first dive was conducted on 26 October 1948 with Auguste Piccard on board. It only descended to 25 m (82 ft), but proved that the vessel's design worked as intended. A second test dive was made with no-one on board on 3 November, which saw the sub descend to 1,338 m (4,554 ft).

On surfacing from its second dive, however, the FNRS-2 was battered by a squall, and its lightly constructed and awkwardly shaped floatation hull was badly damaged. The planned series of dives had to be abandoned, and the Scaldis returned to Belgium having broken no records. The Fonds National de la Recherche Scientifique sold the FNRS-2 to the French Navy, who extensively rebuilt it (redesigning the floatation hull to withstand a wider range of sea conditions) and would go on to use it throughout the 1950s as the FNRS-3.

Piccard, meanwhile, moved on to a new and improved design – the vessel that would become the Trieste. This was to have a larger, stronger pressure hull (made from forged rather than cast steel) and a more robust, ship-shape floatation hull.