Rolex introduced the Sea-Dweller in 1967 as a Submariner developed for saturation divers, and its defining feature is the helium escape valve on the left side of the case, patented that same year. The watch was rated to 610 metres (2,000 feet), carried on a 40 mm steel Oyster case with a black dial, plexiglass crystal and no Cyclops magnifier. The first version is known as the “Double Red” because two lines of red text — Sea-Dweller and Submariner 2000 — sit above the depth rating on the dial.

The valve exists because saturation divers live in a pressurised chamber filled with a helium-oxygen mix: helium molecules work their way into the case, and during decompression they would otherwise force the crystal off. The one-way valve lets the gas escape as ambient pressure falls. Rolex developed the watch with professional diving outfits working at depth, and the Sea-Dweller became standard equipment alongside the Submariner for commercial and military divers through the 1970s. The Double Red dial exists in several generations (Mk I to Mk IV) distinguished by dial printing, depth rating text and valve markings, which is a collecting field of its own.

Rolex built the 1665 from 1967 until 1977, then replaced it with the 16660 rated to 1,220 metres — the depth figure that has defined the Sea-Dweller line ever since. The earlier 610-metre Double Red is now the most sought-after vintage Sea-Dweller: clean examples with original dials, correct hands and untouched cases command strong premiums at auction and from specialist dealers, and the model sits alongside the Milgauss and the Comex Submariners in Rolex’s professional-tool canon. The Sea-Dweller remains in the current catalogue, still with a helium escape valve.