sentences:  Today's Sea-Dweller models are available in steel or steel and yellow gold, and have a 43 mm case.The Deepsea model comes in a 44 mm case in steel and is equipped with the brand's patented Ringlock system, which was designed to provide a higher degree of pressure resistance.During the 1960s, the needs of professional divers working at great depths led to the development of the first 'ultra water resistant' tool watches designed for conducting safe diving operations at 300 m+ (1,000 ft+) depths.