BLANCPAIN | FIFTY FATHOMS, A STAINLESS STEEL AUTOMATIC CENTER SECONDS WRISTWATCH CIRCA 1965

Blancpain's Fifty Fathoms was launched in 1953 and has since defined the specifications and qualities of the modern diving watch. Made at the request of the French Navy, Captain Robert Maloubier and Lieutenant Claude Riffaud provided technical specifications for a watch designed for combat divers. The design revolutionized the unidirectional bezel, which ensured divers could not inadvertently turn the bezel backwards, thus threatening their air supply. 

Blancpain added their own critical feature, a humidity indicator which showed blue if the air in the case was dry. If water penetrated the case, the indicator would turn pink as a warning to the diver. The Fifty Fathoms was named after the British measurement of 50 fathoms, or approximately 91.45 meters, considered the maximum depth which a diver could achieve with the oxygen mixture available at the time.