UNDERGROUND NEW JERSEY

Out where New Jersey nestles against the Delaware River, the fields stretch out and begin to fill with cornstalks that offer the first hint of the miles to the west. And just up from the fields, in an abandoned limestone quarry near Flemington in Hunterdon County, are two narrow culverts that give the first indications of an underground universe that stretches to the Mississippi.

At least I was trying to convince myself of that in the gathering twilight one recent evening as I looked at the opening of a plastic [steel] pipe extending from a solid rock face. After putting my head in, I had to hunch my shoulders to the roundness of the pipe and inch along on elbows and knees, into Leigh Cave.

Yes, cave—for those who thought the only place where one could travel underground in New Jersey were the Holland and Lincoln Tunnels. It turns out that we have more than 100 caves, with antic names like Pipsqueak and Vulture and Crooked Swamp, nearly all in the glacial and volcanic rock formations of the state’s rugged northwestern corner.

This is not an adventure for the claustrophobic. Leigh Cave is the state’s largest and most accessible. (Most of the others are small and on private land.) But it still has spots to make one envy the turtle’s ability to pull its head into the shell.

And because it’s all too easy to be lost or trapped in a bewilderment of dark, narrow passages, it is essential to go with experienced cavers. You can find out who they are—even join one of the three New Jersey chapters, or grottoes, of the national organization for cavers—by calling the National Speleological Society at it headquarters in Huntsville, Ala.: (205) 852-1300 [www.caves.org].

But all these notes of caution do not diminish the allure of caving. Like more sedentary indoor sports, it is something you can do year-round, in nearly any weather: a constant temperature of about 55 degrees makes a cave feel cool in summer and warm in winter. It is individually challenging, yet cooperative. It lets you boldly go where few have gone before. It may be the ultimate indoor adventure—a lot like rock climbing, only in the rock.

ROOM WITH A VIEW

After the pipe at the lower entrance to Leigh Cave comes a five-foot crawl, two inches at a time, into deepening darkness. Then the entrance itself: a small room, its ceiling formed by Precambrian gneiss and its floor of Kittatinny limestone with a generous covering of clay, although the only way to tell is to have a miner’s helmet with an electric or carbide light.

In a few feet, I had a chance to stand up and stretch for a short, careful leap across a 23-foot-deep pit. I was with four experienced cavers and one other first-timer. One caver, Paul Steward of Ewing, wedged himself against a stone projection two or three feet below, to catch anyone who misstepped. Our group leader, John Tudek, of Neshanic Station, the 22-year-old president of the Central New Jersey Grotto, had warned me about all the points where first-time cavers might freeze in their tracks.

We continued, half stopping and half crawling, sometimes sliding on our backsides, in absolute obedience to another dictum of caving: always keep three points in contact with the rock. I had been advised to wear kneepads, one of the single best tips I have ever been given.

Our progress was slow and deliberate, gauged not to overtax the novices. But even that pace seemed like a headlong rush when we came to the Mail Slot. There the passage narrows to barely a foot deep. Mr. Tudek had made this sound like the worst part, but after the entrance pipe, it seemed spacious.

Besides, getting through it offered an immediate reward. Ducking through an opening, we came into the centerpiece of Leigh Cave, perhaps the most spectacular underground space in New Jersey: the Dome Pit Room.

Fifty-seven feet below ground level, we stood on the floor looking up into a balconied room of 25,000 cubic feet, about the size of a three-story Victorian house. An experienced caver, Gene Russo of Toms River, climbed an exterior route to a balcony 30 to 40 feet above the floor. Robert Augustinus of Hillsborough took a narrow internal passage to the same balcony.

New Jersey was lazy about lifting itself out of the ocean, and southeast of a line from Trenton to Bridgewater, its geology is that of sand, silt, and clay—not conducive to cave formation. Even some existing caves were filled by sediment from the last Ice Age. That’s why the state has relatively few caves, said Richard F. Dalton, senior geologist in the New Jersey Geological Survey.

But northwest of that line, the rock formations date to the Precambrian era, two billion years ago, and the Paleozoic, up to 600 million years. Water eroding the soft and porous rock left networks of underground holes and passages that are a delight to geologist and cavers.

And there may be more. Surprise Cave lies near Port Jervis, N.Y., just north of New Jersey’s northwestern corner, and “there is nothing like it from here to West Virginia,” Mr. Dalton said. He added that the rock from which it was formed comes into New Jersey, and that while nothing has yet been found in it, a cave up to a half a mile long could still be found in the state.

HUMIDITY: 100 PERCENT

For now, Leigh Cave, with 800 feet of passages, will have to do. And it does, very nicely. We rested in the Dome Pit Room, sipping water we’d brought and taking pictures; the humidity is nearly 100 percent, which means holding your breath and asking your companions to hold theirs, unless you really want a portrait of steamy clouds.

The return trip was a refresher course on mud and an instant primer on gravity. Going up through the Mail Slot, for instance, was a lot harder than going down through it. And when the path leveled out, it leveled into a puddle surrounded with mud that now covered all of us, front and back, from shoulder to toe. At some point, handholds and footholds were not obvious to a novice, but with tips from the cavers the crawl turned out to be nothing more than good exercise—inching along in an area so close to the surface that the temperature seemed equatorial.

In fact, I was almost ready to turn around to go back to where it was cool. But good sense prevailed, and I crawled through the culvert, four and a half hours after we started, back into a steamy New Jersey night.