Sitting on the gunwale in a pitching sea, 80 feet above the reef, a diver wriggles into his tacky rubber suit and heavy fins. After spitting in his oval mask, he awkwardly shoulders a cylinder of compressed air, cinches his belt of lead weight and puffs in his twin-hose regulator. On one wrist, a compass and depth gauge; on the other, a dive watch, its luminous dial soaking up the tropical sun’s rays. He glances over his shoulder one last time, then reaches down to spin the rotating bezel on his watch, aligning its zero marker with the minute hand, then presses his mask to his face and rolls back into the Caribbean. This is scuba diving, circa 1957.

The rotating bezel is the hallmark feature of the dive watch, recognizable from a distance and so elemental that it seems like it has always existed, evolved like the perfect dorsal fin of a pelagic predator. But in fact, this simple component first made its appearance on underwater watches in the early 1950s out of necessity, at the behest of those early scuba divers who needed a way to track their bottom time. Since then, the dive bezel has changed, been improved, taken on myriad forms and now, ironically, is scarcely used for the purpose for which it was devised. But even though digital dive computers have largely supplanted analog watches on wrists of divers, the dive watch and its signature feature remain as popular as ever, more as a symbol of adventure and rugged functionality.

There were diving watches and rotating bezels well before the 1950s. Rolex put a large rotating bezel on its ultra-rare Zero-graph in the 1930s and it was during that decade that Officine Panerai was selling sturdy underwater watches to the Italian Navy for its combat divers. But the first truly purpose-built diving watches to feature rotating bezels debuted in 1953, when Blancpain, Rolex and Zodiac all introduced watches that would become the archetypes for all the diving timepieces to follow. So why did the rotating bezel become de rigueur for diving watches, a feature that even made its way into ISO 6425, the international standard that governs what can be considered a dive watch? To understand that, perhaps it’s best to step back and look at how and why these timing bezels are used in the first place.

“Bottom time,” or the time spent underwater, is critical for a diver to track. This is because there is a maximum number of minutes a diver can remain at each depth before a buildup of compressed nitrogen in his body tissues exceeds safe limits. If this happens, he cannot ascend directly to the surface without pausing partway up to decompress, or “off-gas,” the nitrogen. Hence, a diver must pay attention to the “no-decompression limit” for each depth. A common mnemonic is the “120 Rule” which states that 120 minus the maximum depth (in feet) will equal the number of minutes that can be spent there. So on an 80-foot dive, the no-decompression limit is 40 minutes, as read off of his watch’s bezel.

Of course, should a diver exceed the no-deco limits, he must pay an underwater penalty of sorts and remain at different depths for several minutes on his way to the surface (and hope he has enough air in his tank) to decompress. These intervals also need to be tracked by the watch and the bezel is once again called into action for these shorter time frames. For this purpose, it is the hashed minutes demarcated on most bezels’ first 15 or 20 minutes that become useful.

Nowadays, digital wrist-worn computers do all of this math for a diver, but before the late 1980s, the rotating timing bezel on a watch was still state of the art, as it had been for over 30 years. For an industry that prides itself on movement complications to solve problems, the rotating bezel is brutally simple. Yet, perhaps it is the watchmaker’s desire to tinker and improve that even though those first rotating bezels did their job adequately, history has seen many variations on the theme.

Blancpain was the first company to make its timing bezel unidirectional, only ratcheting counter-clockwise. A unidirectional bezel is useful since, should it get bumped during the rough and tumble of diving, it will only subtract time from a diver’s bottom time and not put him in danger of overstaying his no-deco limit. Until Blancpain’s patent ran out on this feature, other brands had to make do with bezels that spun both ways. Today, unidirectional bezels are virtually universal.

Another method of protecting a bezel from accidental movement is to lock it and this has been done in several innovative ways over the years. In 1970, Omega debuted its Plongeur Professionel, the mighty “Ploprof,” which featured a massive monobloc case with an odd red pushbutton on one side. Depressing that button was the only way the bezel was free to turn and the trick was (and still is with the modern version) to do it with one hand, using the thumb and two other fingers.

In the late ‘60s, several watch companies decided that the best way to protect the timing bezel was to move it under the crystal. This not only kept it from getting knocked, it also put the elapsed time markings closer to the dial and hands. To control the ring, an extra crown was needed and one of the iconic forms of dive watch was born—the twin-crown diver, a style that has remained to this day in watches like Alpina’s Seastrong Heritage or the Longines Legend Diver. The twin-crown design is an attractive one but not the most ergonomic for real world use since it requires twisting a tiny crown, often with gloves on.

Some brands have taken bezel protection to an extreme by combining the internal timing ring model and adding a crown lock. The Hublot Oceanographic is a technological tour de force with a central minute counter chronograph, massive titanium case and 1,000 meters of water-resistance. The bold timing ring sits under the sapphire crystal and is controlled by a crown at 10:00 that locks in place with precision cut meshed teeth and capped with a hinged lever. While well protected, it also presents a challenge for the diver when it comes time to operate it, perhaps a case in point of the best solution being left to the simplest one.

IWC is known for its over-engineered dive watches and since its first one, the 1967 twin-crowned Aquatimer, the Schaffhausen brand has had an impressive lineup of subaquatic bottom timers that have sported both internal and traditional external timing bezels. The latest in the Aquatimer lineage makes use of a groundbreaking technology that IWC calls “SafeDive.” The timing ring is under the sapphire crystal but instead of being manipulated by a second crown, it is operated by turning an external bezel.

This outer grippy ring is linked to the inner rotating scale by a geared clutch mechanism on the left flank of the watch. Though the bezel spins both ways, the inner ring only ratchets counter-clockwise. Though complicated in design, it is incredibly easy to use and is a solution worthy of IWC’s legendary engineering heritage.

External bezels remain the most ergonomically designed but due to their location, they can be vulnerable to damage. While it is rare, in some cases a bezel can pop off a watch from a direct blow; while this isn’t a common occurrence in recreational diving, for commercial divers who work in more extreme environments, it can happen. In the mid-70s, Seiko responded to this problem by surrounding the bezel of its 600 meter Professional dive watch with a tall shroud that was screwed onto the watch case. While initially this was made from metal alloy, today, Seiko’s top of the line 1000 Meter Marinemaster features a shroud made from hard ceramic. The bezel remains accessible at two diametrically opposed finger positions but perfectly protected otherwise. This shroud gives the Seiko Marinemaster 1000 Meter a unique appearance, unlike any other dive watch, and one that inspired the watch’s nickname, the “Tuna Can.”

Besides protection and manipulation, another innovation related to dive watch bezels has been in the markings featured on them. While traditionally this has consisted of a simple one-hour hashed scale to track elapsed time, in 1967, Doxa introduced a bezel that had the no-decompression scale engraved on it, in addition to the usual time markings. The maximum depths for each no-deco time up to one hour were marked on the outer ring, providing a sort of crude “dive computer” at a glance, and this bezel became Doxa’s calling card, still present to this day on its line of SUB diving watches.

Aside from their markings, manipulation and protection, bezels have evolved since their first appearance in terms of their materials as well. The earliest ones were simple printed aluminum rings, pressure-fitted in place. Some were made from Bakelite, an early form of plastic that was susceptible to cracking, while others, like the Doxa’s, were made from tough, engraved steel. The weakness of all of these materials is in their susceptibility to fading and scratching, so in the past decade, brands have come up with new materials from which to craft their bezels. Sapphire came first, followed by ceramic, both virtually scratch- and fade-proof. Omega, meanwhile, has made use of a proprietary material called LiquidMetal that has all the advantages of ceramic but allows for more flexibility in color and appearance.

In 2017, dive watch bezels may not be used much for timing dives or tracking no-decompression limits. But they can still be useful to a diver, such as in navigation, when timing a swim distance is crucial, or for tracking the time between dives (the “surface interval”). And of course, should a battery-powered dive computer fail, on the wrist of a diver who remembers his “120 Rule” a traditional dive watch can still save the day.  But beyond these uses, the dive watch is the calling card of the diver, a symbol of this community of adventurous people who still explore the world underwater. And the feature that makes a watch a dive watch is, of course, the rotating bezel.