First, an admission: I have mixed feelings about writing this. As a cave diver, I understand the desire of cavers and open-water divers to explore the depths of submerged caves, but all too often, gear columns encourage the inexperienced to think that equipment is the key to quantum-leaping their skills. Perhaps nowhere else can this mindset be more dangerous than in cave diving, which demands experience and knowledge as much as equipment if you're to stay in the game for any length of time.
One of the most prolific of all cave explorers, the late, great Sheck Exley, identified five main causes of cave-diving accidents, and these all subsequently had a significant effect on cave diving and its equipment. These are: exceeding the level of one's training; the lack of an underwater guide line; the effects of depth (something that may not always be apparent in an underwater cave); an inadequate supply of air; and insufficient light sources.
So, first and foremost, the most critical thing to lay your hands on is good training. Whatever your background and interest in caves and diving, make sure that you learn properly. Join a caving club, learn about caves, learn to dive and seek proper instruction through organisations such as the UK's Cave Diving Group or the USA's National Association for Cave Diving. When things go wrong underground and underwater, thorough training and a cool head is what will get you home alive, not the latest piece of gear.
MEANS TO AN END
The choices in cave diving equipment were originally driven by the type of cave to be explored, whether they be springs or sumps. Access to spring caves is easily gained from the head-pools of springs, and the original spring divers were mainly open-water divers attracted by the notion of exploration of the caves below. This, along with the fact that spring caves tend to consist of big passageways, meant that open-water diving techniques and equipment could be readily adapted for spring-cave diving. Florida, the Dordogne and Mexico's Yucatan Peninsula are all renowned for their exotic spring diving.
By contrast, sumps are found at the end of terrestrial caves where the roof meets the water level. The original sump divers were cavers who used diving as a way of passing the sump to explore the dry cave systems beyond. So, whereas diving was central in the development of spring exploration, in sumps it was merely a means to an end. Sump diving is best described as caving underwater and usually takes place in much smaller passages where a typical spring-diving equipment rig can't be used. Swildon's Hole and Wookey Hole are two sump caves in Somerset whose history is bound up with that of cave diving.
Whatever type of cave is being explored, the essential tools of the trade are reels, which all divers will use to lay a permanent guide line to the surface. Although cave divers may use different reels with different lengths and types of line depending on the cave, all divers will carry a search reel, which is the main tool a lost diver will use to find the main line in a cave. This is arguably one of the most important drills a cave diver will ever master.
The 'rule of thirds' was developed to provide for enough breathing gas. This allows for a third of your air to be used on the inbound journey, a third for the return, and a third for use in an emergency. In certain circumstances, divers may choose to be even more conservative.
In spring diving, the diver typically carries, their breathing gas in back-mounted twin cylinders linked by manifolded valves. This means that much larger cylinders are used than in open-water diving. Divers may also carry additional 'stage' tanks to allow further penetration into a cave. Whereas a single ten-litre cylinder is a common size in open-water diving, it's not uncommon to see cave divers carrying two back-mounted 20-litre cylinders and several ten-litre stage tanks on some longer dives. …