During the week, Russell Kennedy works as a cartographer for the National Ocean Survey. He often draws maps on the weekends, too -- but a course of a different color: the dark insides of caves.

On most Friday nights, Kennedy loads up the car and takes off with friends from one of three caving clubs he belongs to. On a recent exploration in West Virginia, members of the Potomac Speleological Club spent 15 hours in a cave -- three hours walking, crawling and climbing to its uncharted section, nine hours surveying and three hours returning to the entrance.

During those final hours, says Kennedy, "you're crawling as fast as somebody could possibly crawl. You reach a point where it's time to get out."

It was 3 in the morning when they emerged, but the foursome -- cavers always travel in fours for safety -- sat up the rest of the night talking about it.

"We only surveyed 867 feet," says Kennedy. "That's kind of embarrassing. I've been on trips where we've surveyed 2,000 feet in less time."

In addition to exploring and mapping cave systems, serious sport cavers like Kennedy also put gates over cave entrances -- either because the property owner requests it, or because the cave houses endangered animals such as some species of bats. Conservationists by nature, they also lobby for legislation to protect caves.

As chairman of the Sligo Grotto of the National Speleological Society, Kennedy is involved in all of those activities. But caving goes deeper.

"There's no doubt that it's a religion for some people," he says. "They go every Sunday."

Caving is practically Kennedy's whole life.

"To me it's another one of God's wonders," he says. "The same kind of thing a monk would experience, or someone involved in zen: doing something hard, to get so involved in what you're doing that it's almost like meditation.

"When you are underground, you're totally cut off from everything that you're used to. There's no real weather, just sometimes wind blowing through spots. There's no rain except the drip-drip-drip from the ceiling occasionally." (It may be freezing outside, but cave temperatures remain in the 50s.)

"There's something wonderful about being way underground with no light but the light you've got. It's quiet, except for the people with you.

"Caves are hard to describe. Even if there are no formations, they are beautiful places. It's the solitude. Some of them are like big jungle gyms underground. There's a little kid in you who always wanted to play in the mud and crawl around in the dark."

Most people hereabouts are familiar with what Kennedy calls "show caves" -- such as Luray, Massanutten, Skyline and Grand Caverns in the Shenandoah Valley.

"The prettiest caves are open to the public; there's no doubt about that," he says.

But the sport of caving is more an acquired taste. When Kennedy first started in 1972 at the urging of college roommates, he encountered mainly fear.

"Two guys took me caving and left me sitting for 45 minutes on a ledge, with my light getting dimmer," he recalls.

Caving is not what the average citizen considers fun, says Kennedy: "It's dirty, it's tiring, it's muddy, it's kind of dangerous, though if you have the proper training it isn't dangerous."

Cavers should know climbing techniques and such survival skills as dealing with hypothermia, best learned through a caving club.

"Caving is not a showoff sport like mountain climbing," says Kennedy. And if there's an accident in a cave, it may be harder to reach the injured person, who might be down past two or three crawlways with a broken leg.

As an offshoot of caving, Kennedy is hooked into a network of cave rescuers. On a recent Saturday, he joined a mountain rescue group to learn even more about search and rescue.

The cave maps that Kennedy helps to draw may be used in a rescue. But besides that, they mark where someone has been.

"If you've explored a cave and you haven't mapped it," says Kennedy, "how can you say you've explored it?"

The ultimate would be to find an undiscovered cave. This may have happened to Kennedy in 1975: "I felt I was in a place nobody had ever been before," he says. "It's like climbing a mountain, to be the first to the top."

Although Kennedy describes himself as slightly claustrophobic, he doesn't mind caves, even tight spots. It's the close sleeping bag that gets to him sometimes.

"I'm not afraid of dying, either," he says, but adds: "I still jump at things, if all of a sudden an insect drops on my neck. I'm afraid of being injured, but I don't think about it. I worry more about running out of energy when it's time to go out," says Kennedy.

Cavers often become good friends because at times their lives depend on one another, Kennedy believes. The friendships made through caving mean a lot to him.

"I have neglected my non-caving friends to where they rarely see me," he says. "But then I have less and less in common with them. I may be obsessed with caving, but I know so many others who are."

Kennedy, 33, doubts he'll ever get married. "I'm too busy caving. My social life tends to suffer." But, he says, "My whole social life revolves around cavers. Admittedly they are men and women -- so, who knows?"

For things above ground he has little interest. Another weekend pursuit has been rugby -- more refereeing in recent years than playing -- but he's taking a break from it.

"There's pressure to excel even when you are referee," he says. "But caving doesn't have that competitive pressure. Maybe I'm trying to escape from competition."

Kennedy collects what he calls "junk -- minerals, maps," but when asked if he collects things in caves, he quickly replies, "Oh dear, no! Verboten! You don't even consider it!" He cites the motto of the National Speleological Society: Take nothing but pictures, leave nothing but footprints . . .

"That's why we don't tell people where our caves are, because some rockhounds would tear it apart," he says. "I would never want to be caught with anything in my house that could have come from a cave. Stalactites, stalagmites -- most of the time when you take something out of a cave, it loses its beauty.

"A live cave is something that still has formations forming. The oil from your fingertips will turn a white formation black. You avoid touching things. You can touch the mud.

"They have been thousands of years forming. You want to leave things for future generations. You never write on the walls of the cave. You don't write arrows to show the way; you leave a popsicle stick, pick it up when you go out. String, that's an old joke. You never have enough string to go through an entire cave anyway, not that you could carry.

"Caves are one of the least accessible wilderness areas of the world -- yet in some cases within half a day's drive of major cities in the U.S. -- and we feel we have to protect them."