Before they return to the open air, professional divers heading for the surface after a deep saturation dive must spend time in a decompression chamber, where they breathe a gas mixture containing helium. The tiny molecules of helium, an extremely light and non-volatile gas, infiltrate everywhere in the chamber, also penetrating the watch. During decompression, the helium is unable to escape from the waterproof case quickly enough, creating a pressure differential that could force the crystal out of the watch case. Rolex engineers created a gas escape valve fitted with a spring: it opens when the difference in pressure between the inside and outside of the watch reaches 3 to 5 bars, allowing the helium to escape, thereby protecting the watch.