Few diving experiences compare with finning to the mouth of an underwater cavern or cave, putting your hands on the lip and peering into an open chamber filled with formations, shafts of light streaming from holes in ceiling, illuminating sections and leaving others draped in shadow. The names of such places--the Cathedral, the Ball Room--barely do them justice in their attempts to capture these images. All dive-training agencies have limits in place regarding divers' access to overhead environments--and for good reason. "Since the 1970s, more than 600 divers have died in overhead environments," says Jeff Bauer, President of the National Association of Cave Diving (NACD). "And most of those deaths happened because the divers didn't have the appropriate training." But with the right training, you can add a whole new dimension to your diving adventures. The first step is a cavern-diver course; most recreational dive-training agencies offer this class in a form derived from the curriculum taught by cave-diving organizations NACD and the Cave Diving Section of the National Speleological Society (NSS-CDS). These classes provide the knowledge and techniques needed for limited penetration into overhead environments, they introduce you to new types of equipment, and they help fast track development of the most vital underwater skills--like buoyancy, trim, air consumption, swimming techniques and safety procedures.

Defining "Cavern," and Course Prerequisites
Different training agencies have slightly different definitions for what constitutes a "cavern" dive, but generally, across all agencies, it is defined as a dive in an overhead environment--like the mouth of a cave--where two divers can easily swim side-by-side and remain within the natural light zone. "You have to stay within the daylight zone," says John Jones, training director for NSS-CDS. "In our courses, we also specify a maximum depth of 100 feet, and cavern divers can't go more than 200 linear feet from the surface."

The experience level required to enroll in a cavern course also varies among agencies, but in most cases, an Advanced certification and/or 15 to 20 logged dives are a minimum. More important, students in a cavern course need to demonstrate above average buoyancy control; cavern instructors will evaluate students during an open-water dive before going into the cavern to make sure each diver is ready to go inside. "A couple of times I've had to tell students, 'you need to go practice buoyancy and come back,'" Jones says. "But probably 95 out of 100 divers make it through the class."

The Gear
Because cavern dives are defined as within recreational diving limits, they can be made with recreational dive gear. "We try to give divers a taste of the methods we use as technical divers without them having to buy a whole lot of extra gear," Bauer says. "In a cavern class, it's OK for them to use their standard open-water BCs, single tanks, all the stuff they're used to--obviously they don't need their snorkels though, because you can't come straight to the surface and it's an added entanglement risk."

Dive Lights
You stay within the sunlight zone, but to see every crack and crevice along the way, you need to throw some beams of your own. Bring at least two. For cavern diving, standard night-diving gear will do the trick, Jones says. "If you're set up to night dive, with the standard primary and small secondary for backup, you're alright for cavern diving," he explains. However, cylinder-shaped lights, or lantern-grips, are the best choices because it's easier to handle a reel and guideline with one of those than with a pistol-grip light. And if you really want to illuminate like a caver, pack a hand-mounted light attached to a large battery canister, which is generally mounted on the waistband of a harness or on your tank band. "I'll usually let students borrow one of my canister lights," Bauer says. "Just so they can get the feel for a brighter light that attaches to their hand."

**Reels
** All recreational cavern courses include basic line-laying techniques. Even within the light zone, a misplaced fin kick or a bad-buoyancy crash on the bottom can blow out the vis. And when you can't see, a reel literally acts as your lifeline back to the exit. At minimum, each diver in a cavern course needs a small safety reel, and each buddy team needs one primary reel.

Cavern Skills
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Buoyancy
** The first order of business in a cavern course is to figure out the ideal weight each diver should carry. "Most people come to us overweighted, so we help the students find the perfect amount of weight they need," Jones says. "Generally, we take four or five pounds of weight off every student in a cavern class. I don't even remember how much weight I had on when I took my cavern course, but from what I learned in that class, and after switching to steel tanks, I got rid of my weight belt entirely." Ideally, cavern divers can hover motionless in the water and effortlessly maintain a proper swimming attitude. This is critical because if you float too high, you ram into the ceiling; drift too low and you kick up the bottom. Either move can turn gin-clear water into silt-choked sludge in seconds.

Weight Redistribution
The problem with a normal recreational dive setup is that all your weights hang around your waist, and you compensate with air at your shoulders. This puts you in a poorly streamlined, head-up/feet-down position, requiring more energy (and air) to move through the water, and increasing your chances of kicking up silt along the bottom. Once you know how much (or little) weight you need, distributing it properly greatly improves your trim. In a cavern course, you get a chance to think outside the pouch. Integrated BCs have trim pockets in the back that help move some lead higher up on your body, and your cavern instructor may also attach small weights at your shoulders, Jones explains. Making the switch to steel tanks--which, unlike aluminum tanks, are negatively buoyant--helps you eliminate extra bricks all together. And though it's not required in a cavern course, upgrading your BC to a tech-style backplate and wing also adds inherent weight along your torso, eliminating even more lead bricks.

Laying a Line
You'll practice line techniques on land and in open water with your buddy before laying one out in a cavern. "They'll learn how to tie off a reel to a fixed object and how to route the line on the bottom so it doesn't get trapped or wrapped up easily," Bauer says. This task is surprisingly difficult for the uninitiated. You have to run the reel with your buddy so it can be followed in zero visibility without getting tangled and while also holding a light and maintaining perfect buoyancy. You'll also practice following a guideline--with your eyes closed to simulate blackout conditions--both as a normal buddy team and while donating and receiving an alternate air-source with your buddy to simulate a worst-case emergency scenario. "It really teaches you how to multitask," Jones says. "It's probably the most intense two- to three-day course you'll ever take."
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Gear Configuration**
Redistribute your weight, reroute hoses, reverse fin straps and make sure no accessories are dangling. Why? Because anything that dangles can foul the guideline. "Everything should be secured," Bauer says. "At the cavern level, I'll make sure divers clip consoles across their chests, replace the lanyards on their lights with clips and duct tape fin straps down." The end result is a streamlined package that not only minimizes tangles but that also helps you move smoothly through the water. "The nice thing about the cavern class is that the environment usually teaches this stuff for me," Bauer adds. "I can point out things that need to be reconfigured, but when the divers go down and get tangled in the line, they quickly realize the importance of getting rid of any dangling equipment."

The Kicks
Streamlined movements and strong, efficient fin strokes that don't kick up the bottom are key in caverns. First up: The frog kick. Keep your knees bent, and start by spreading your feet apart with the narrow side of the fin cutting through the water. Then turn your ankles so the bottoms of your fins face each other, and bring the fins together like two hands clapping. This pushes the water between the blades and directly behind you, unlike a flutter kick, which forces water up and down, possibly stirring up the bottom. Next lesson: Mastering the fin turn. Instead of flapping your hands to turn your body, a few well-placed fin flicks can spin you in the right direction with less effort and less vis-destroying turbulence. To do this, float motionless in the water in a normal, face-down swimming position, but with the knees slightly bent. Imagine your navel as a pivot point, and turn your body with short, inward flicks of the foot opposite the side you want to turn to--i.e., use the right foot to turn left and vice versa.

Can't-Miss Cavern Dives
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Gran Cenote, Riviera Maya:** The systems of cenotes that perforate the land along Mexico's Yucatan coast comprise the largest underwater cave systems in the world, and cave divers flock from around the world to dive here. But you don't need to be a full-fledged cave diver to experience some of the best Riviera Maya has to offer. Gran Cenote, one of the most popular, starts in an open chamber accessible even to snorkelers and the mouth of the cave provides perfect conditions for cavern diving.

Ginnie Springs, Fla.: Second only to the cave systems at Riviera Maya, the underwater rivers that make up Florida's spring systems are an equally great place to get your cavern diving fix. Ginnie Springs features a wide-open chamber called the Ballroom, and metal grating blocks passage to the deeper, more dangerous parts of the cave.

The Cathedrals, Maui, Hawaii: For a cavern experience that's totally different than those offered in Mexico and Florida, check out the lava formations of the Aloha State. Two of the most famous cavern dives here are Cathedral One and Two, off the island of Lanai. Both feature wide-open chambers, dramatic rock formations against a backdrop of blue water and multiple entry and exit points.