Inside Rolex’s Gruelling, Top-Secret Dive Watch Testing Facility 

–Chris Hall

Rolex watches have survived missions to the very bottom of the Pacific Ocean. This is how the company tests 800,000 watches a year without leaving Geneva.

There is perhaps no family of watches subject to more intense levels of fandom than dive watches, and no area so riddled with debate and misinformation than water-resistance. The very concept of a diver’s watch might seem quaint and arcane, and it’s true that professional divers have been leaving their mechanical watches on dry land in favour of wrist-borne dive computers since the late 1970s. But before that, a dive watch was a tool that could save your life, and its design and manufacture was carried out with all the gravity that implies. Buy one today and you are still connected, however loosely, to that era of exploration, adventure and endurance.

To be described as a diver’s watch, the timepiece must comply with ISO 6425, which sets out minimum requirements for resistance to water pressure, shock, corrosion and magnetism, stipulates that it must have a unidirectional rotating bezel to measure elapsed time, and be sufficiently legible in complete darkness. Most importantly of all, ISO 6425 defines a dive watch as being able to withstand at least 100 metres of water pressure. Below that, any claims of water resistance should not be taken literally – a watch rated to 30m or 50m is something you should take off to do the washing up, let alone wear for a swim.

Rolex produces in the region of 800,000 watches every year, and while the exact figures are a closely guarded secret, a significant percentage of that number will be dive watches. Of these, the majority will be Submariners, with a smaller number of Sea-Dweller and Deepsea models alongside those. A Submariner is water-resistant to 300m; a Sea-Dweller to 1,220m and a Deepsea to 3,900m. (It is also worth pointing out that absolutely every Rolex Oyster model – that is, every Rolex save for the Cellini family – is water-resistant to 100m, even the most delicately gem-set Pearlmaster or luxurious Day-Date, and it has been that way since Rolex produced the world’s first truly water-resistant watch case, the Oyster, in 1926).

In 1960, just six years after launching the Submariner, Rolex set a dive watch record that still stands when it sent a one-off custom-made watch, the Deep Sea Special, to accompany the bathyscaphe Trieste to the bottom of the Mariana Trench. Strapped to the outside of the submarine, it was 36mm thick and resembled a glass eye affixed to a metal bracelet. It withstood 10,916m of pressure. In 2012, when filmmaker James Cameron became the first to undertake a solo expedition to the same depths, Rolex once again took the opportunity to demonstrate its capabilities, creating the Deepsea Challenge, a 51.4mm monster rated to 12,000m. At this depth, it was bearing 12.35 tonnes of pressure – more than the weight of his Deepsea Challenger submersible itself.

All of which begs the questions: just how do you engineer watches to take such pressure? And more to the point, how do we know they really can? In the first instance, we must begin with the materials. Rolex uses 904L stainless steel (most watch brands use 316L) for its greater resistance to corrosion and ability to take a finer polish. Its dive watches have titanium casebacks – for their hypoallergenic properties as well as material strength, and where the sapphire crystal over the dial has a “cyclops” date magnifier (as the Sea-Dweller now does, since 2017), it is milled from a single piece of sapphire crystal to eliminate any structural weaknesses. Fun fact: the sapphire crystal protecting the dial on the one-off Deepsea Challenge is thicker than an entire Rolex Submariner (14.3mm vs 13mm).

Then there is the design. The exact details which prevent a Rolex dive watch exploding under pressure are not all revealed to the outside world, but suffice to say 65 years of making dive watch cases has made its mark. Both the Sea-Dweller and Deepsea use a triple-sealed screw-down crown (the greatest point of weakness for a dive watch) and the Deepsea employs a patented case construction that uses nitrogen-alloy steel within the 904L steel and titanium outer case that allows it to dive three times deeper while only being one millimetre wider (44mm vs 43mm) and 2.2mm thicker (17.7mm vs 15.5mm), and roughly 20 per cent bigger overall. This year, the watch has been tweaked slightly for better aesthetic proportions, giving it a thicker bracelet, sturdier lugs and sides and a correspondingly bigger clasp.

The answer to the second question lies in a 1.3 tonne stainless steel tank, cast in a single piece and sitting somewhere inside Rolex’s Les Acacias factory in Geneva. Created for Rolex to the brand’s own specifications by French maritime engineering specialists Comex, for whom Rolex made watches in the 1960s, it is the hyperbaric pressure chamber through which all Rolex dive watches will pass: a baptism not of fire but of water.

Each watch will already have been subject to a preliminary test for air-tightness before being entered into this tank, wherein the pressure is ratcheted up to 25 per cent beyond the watch’s stated levels of endurance; a Deepsea is tested to 4,875m. (The Deepsea Challenge was tested all the way to 15,000m, deeper than it could ever be asked to go, using an even bigger custom-made hyperbaric tank).

What the tank can’t do is actually tell if there is a leak – that requires a simple but separate test. Once removed from the tank, the watch is heated and a cold metal rod placed on the sapphire crystal; should any condensation that might appear within the watch not disappear after 60 seconds, it has failed the test. Should that happen, it would pass to a vacuum chamber to locate any potential leaks.

It is thought that fewer than 0.1 per cent of all watches show any sign of failure. That hardly comes as a surprise; this test is the last step in the creation of watches that have had every consideration given to their physical strengths. Like a world champion athlete winning gold at the Olympics, it is a test they were built to pass.