Deep sea submersible FNRS-III on display at the French Naval Base at Toulon France. In 1954 the FNRS-III was the first vessel to reach the Abyss, when it went beyond 4 kilometres (2.5 miles) deep off the coast of Dakar, Senegal. It was piloted by Georges Houot and Pierre Willm, officers of the French Navy. Further information in ‘The Frontier Below’.

https://www.jeffmaynard.net/the-frontier-below-book/