About the photo (above):
The base of the mainmast from the Cornelia B. Windiate. The Windiate is a three-masted schooner that sank in 1875 in 185 feet (56 m) of water off Presque Isle, Michigan, in Lake Huron. Photo taken during the August, 2003, archaeological survey organized by Joyce Hayward. The centerboard winch can be seen at the base of the mast with the archaeological baseline passing by it. Mast hoops and a fife rail surround the mainmast. Photo taken by survey participant Rod Maxson.
Technical diving is an advanced form of scuba diving. It involves specialized equipment, training, and gases to be able to safely enter places not appropriate for recreational divers. Typically, these environments involve what is termed an "overhead." The overhead may be a physical ceiling such as in an underwater cave or inside a shipwreck, or it may be a "virtual" ceiling in the form of a decompression barrier that prevents the diver from immediately surfacing in an emergency.
Technical divers carry much more equipment to safely enter these realms. They almost always carry double tanks on their backs with two independent regulators attached. This is done for redundancy in case of the failure of one regulator. The two tanks also allow more gas volume to be carried. The increased gas volume allows both deeper depths to be reached and extra margins of safety. Additionally, smaller cylinders may be carried clipped to the diver's side to aid decompression or add even more gas volume and redundancy. Redundant lights, cutting tools, guideline reels, and surface marker bags may also be carried.
While a large amount of equipment is the hallmark and much maligned feature of technical divers, this is far from the only thing required to successfully perform technical diving. It also requires rigorous training and a clear presence of mind. Not only do divers need to know how to safely perform bailouts when presented with equipment failures or other emergencies, they must also be able to perform the disciplined art of decompression diving. This requires following a rigorous schedule of arrival and departure times at different depths in order to be able to safely reach the surface without suffering from decompression sickness, or "the bends." It also frequently involves switching to high-oxygen gas mixtures stored in decompression cylinders at the proper times and depths. This is one of the most dangerous actions in technical diving - the wrong gas switch at the wrong depth could result in either the bends or a fatal oxygen toxicity convulsion. Additionally, intense training involves using lights to signal, how to properly deploy floating marker bags from depth, how to properly install and use guidelines, how to handle light failures, and much more.
Rarely do the gases used by technical divers involve air. For shallow depths, mixtures involving 25% to 40% oxygen are frequently used to increase bottom time. These mixtures are called "nitrox." To speed decompression, additional mixtures of 50% to 100% oxygen are carried. For deep depths, another gas mixture called "trimix" is used that adds helium to alleviate the effects of nitrogen narcosis. Helium is not narcotic at depth and is used to replace some of the nitrogen. Unfortunately, helium can be fairly expensive and sometimes hard to obtain, making the endeavor of using trimix expensive. Because these exotic gas mixtures are not commercially available even at most dive shops, many technical divers maintain their own gas mixing equipment and compressors to fill their tanks and decompression cylinders.
Despite all of the trouble and expense of technical diving, the rewards are considerable. This form of diving allows divers to safely go to places infrequently or never visited before. Most individuals get involved slowly out of necessity to be able to continue exploring new wrecks or caves. As more experience is gained, the depth and time limits are pushed further back. This has resulted in the evolution of techniques and equipment over the past decade that has revolutionized the sport of diving. It has resulted in a new Golden Age of diving with new frontiers being discovered and explored. Many technical divers now safely and routinely reach depths of 200-250 feet (60-75 m), and some are beginning to reach depths of 300-400 feet (90-120 m) using gas-saving machines called rebreathers.