In the 20th century, great leaps forward in technology brought previously uncharted territories within reach. Wristwatches quickly became vital safety tools. In the oceans, the invention of SCUBA (self-contained underwater breathing apparatus) gave divers unprecedented new freedom of movement to explore the depths. But it also brought new risks.
Dive watches developed quickly to address those risks. Unidirectional bezels (which could not rotate clockwise) were created to make it impossible for a diver to inadvertently knock the bezel during a dive to give a mistaken indication of more time available underwater. Tough cases and crystals, screw-in case backs and screw-down crowns were developed to withstand the water pressure of ever deeper dives. Finally, watch dials evolved into supremely legible displays with large light-emitting indexes that could be read even in low light.