Today, when you execute planned decompression stop dives using mixed gas and multiple gases, with open-circuit or closed-circuit hardware, in the caves of Florida, Russia or Mexico, the wrecks of Scapa Flow, the Great Lakes or Truk, the abandoned flooded mines of Finland and South Africa or the walls of Palau—whichever of the alphabet soup of agencies trained you in this kind of diving—you will be carrying the same equipment, configured in one of a few recognised, accepted and well-tried ways, and using similar combinations of gases, decompression tables and procedures. Simon Pridmore has the story.