The Rolex Sea-Dweller reference 16660, known to collectors as the Triple Six for its 666m depth rating, is the watch that introduced one of the most significant technical leaps in modern Rolex history — the first Rolex to receive sapphire crystal, the first Sea-Dweller with a unidirectional rotating bezel, and the first to use the new-generation Caliber 3035 movement. Produced from 1978 to approximately 1989, the 16660 succeeded the legendary 1665 Single Red and Double Red Sea-Dwellers and brought the line firmly into the modern era while preserving the tool-watch DNA that has defined the Sea-Dweller since its 1967 origin. The 40mm Oystersteel case carries the helium escape valve at nine o'clock, the patented Rolex feature engineered for professional saturation divers operating in commercial dive helium environments. The black matte dial with applied tritium hour markers and Mercedes hands gives the reference its distinctive vintage character, with the depth rating printed at the foot of the dial — the 666 designation that birthed the Triple Six nickname. Powered by the Rolex Caliber 3035, a COSC-certified self-winding chronometer with 27 jewels, the movement that replaced the Caliber 1575 and laid the groundwork for the Caliber 3135 that succeeded it in 1988. Worn on the Oystersteel Oyster bracelet. Pre-owned in vintage condition.