Propulsion while surfaced was provided by two 4,474 kW (6,000 hp) diesel motors, with a maximum speed of 18.6 knots (34.4 km/h; 21.4 mph).

[4] According to Prométhée's commander, the tests on the electrical motors to ensure propulsion while submerged, were completed a little before noon on 7 July.

Prométhée was stationary 7 nautical miles (13 km; 8.1 mi) off Cap Lévi, close to the commune of Fermanville.

While ascending the conning tower, he clearly heard the ballast purges opening, without noticing anyone at the electrical controls.

Enseigne Bienvenue ordered the opening of the purges and the closing of the hatches which allowed passage between the submarine's exterior and interior.

As news of the loss reached shore it became clear to the military and civilian authorities at Cherbourg that any rescue would have to be carried out within a couple of hours.

An investigative committee determined that the crew did not survive more than a couple of minutes following the sinking, after the submarine was completely flooded.

[5] Artiglio sailed from Le Havre, and Rostro from Brest, with the two ships arriving at Cherbourg on the morning of 9 July.

Diving teams from Fidèle and Artiglio struck the hull with hammers in an attempt to contact survivors, but received no response.

International experts were consulted - notably Mario Raffaeli, technical director of the Italian society of maritime rescue recuperation, and Ernest Cox, who supervised the salvage of German ships from Scapa Flow - in order to assess the possibility of refloating Prométhée for investigative purposes and to recover the bodies of her crew.

In the evening of 8 July Vice-admiral Le Dô, maritime prefect of Cherbourg, established an investigative commission to discover the causes of the sudden sinking.

Unable to examine the wreck, the commission was unable to use physical evidence, and had to rely on interviews with the survivors, including the commander of Prométhée, Lieutenant de vaisseau Couespel du Mesnil, and conducting similar trial simulations on Prométhée's sister ship Archimède.

Engineers studied how the Archimède submerged in as orderly and quick a manner as possible, to compare with what had happened to Prométhée.

This unexpected opening could have been due to an error, or connectivity in the oleo-pneumatic section valve systems [clarification needed].

This design flaw was well known prior to the loss of Prométhée: a similar incident had occurred within gravity [clarification needed] on Redoutable and Vengeur.

Lieutenant de vaisseau Amaury Couespel du Mesnil was summoned before the war council on 3 November 1932, in order to answer for the loss of his boat.

A ceremony of national mourning was held on 30 July at Cherbourg and at the wreck site, in the presence of the Council president Édouard Herriot and Naval minister Georges Leygues.

Donations from across France funded the erection of a monument in the form of a cross at the point on shore closest to the wreck site, commemorating those lost.