POLEBRIDGE -- Beneath: a bed of soft sand pressing damp and firm against shoulder blades.

Above: a glittering sky, starry pinpoints against unlimited black, deep as time. All around: the song of the current, a quiet lullaby of water laughing, tumbling, spilling over stone.

Perfectly still. Deceptively peaceful.

So when the star cut loose from the sky, shooting through space, I barely had time to blink before it smacked me in the forehead.

"Wet," I thought with surprise.

We had guessed it might be salts, a shining precipitate clinging to the cave ceiling. Or perhaps tiny glints of gleaming mica.

But it was water, gathered into countless droplets no bigger than the head of a pin, sparkling the rock roof into a Milky Way dazzle.

Who knew? Certainly not the guy with the wet forehead. This was a whole new world, the underworld, and the most knowledgeable among us was an 8-year-old geology kid with his mineral chart and a backpack full of magnifying lenses.

It was his fault that we were lying here in the dark, using just one tiny penlight to make the roof sparkle. He's enamored of rock, of stone and of mineral, of pebble and boulder and crystalline quartz.

Our house is becoming a museum, with carefully labeled rocks stacked shelf after shelf. Rocks tumbling in a polisher, rocks in pockets to be sorted before laundry. Schist and slate and folded gneiss. Gabbro.

What better place, then, than underground? Underground is made of rock. Even I knew that.

We'd heard of these caves in far-northwestern Montana riddling steep limestone mountains, tunneling, some said, right beneath the border to Canada. But we'd never ventured below.

So we hit the library, learned what we could, learned that Montana is, in many ways, a caver's kind of place.

"Montana has a lot of wilderness," said Mike McEachern, "and it has a lot of limestone that no caver has walked on yet. So Montana has vast potential."

If you're a true explorer, he said, interested in making the map instead of following it, "Montana's full of possibilities."

McEachern is a mapmaker, an explorer, a caver and president of the Northern Rocky Mountain Grotto, a caving club chartered by the National Speleological Society. His outfit claims about 50 members beneath the Big Sky, which isn't so big underground.

But McEachern doesn't like talking about particular caves. Asking him the location of an entrance will get you about as far as asking a hunter where he shot that fine bull elk.

The trouble, he said, is a simple lack of respect. People paint the walls, pocket the rocks, break off stalagmites (or are those stalactites?) and blacken the ceilings with fire soot.

"It kind of makes us a little gun-shy," he said, because geology operates on a whole different schedule than biology; that rock formation you just snapped off might have been growing for 10,000 years.

Caves are not, it seems, renewable resources, and so rather than telling you where to go, McEachern would much rather take you along. His Grotto makes several group trips each year, often into the karst topography of the Bob Marshall or Scapegoat wilderness areas.

There, nestled into the limestone cliffs of the Chinese Wall, are some of the nation's most tremendous caves.

There's Virgil the Turtle's Great House Cave, 1,600 feet deep and more than a mile long, the second-deepest limestone cave in the nation. There's Moonray and Sunray and Meanderbelt, Frogg's Fault and Dover's Drop and Coffeepot. Lone Goat and Spinshaft, Reaper's Rock Pile and The Blood Cave.

McEachern was among the intrepid few who discovered and mapped Blood Cave back in the 1970s, hiking miles into the wilderness to explore the sprawling Silvertip system. Combined, Silvertip and Blood caves twist and turn and fork for more than nine miles beneath the surface, a world of such bottomless dark that the blackness actually feels as if it's pressing against your eyeballs.

It took years to hook it all together, he said, to find the many entrances and exits and weave the web of interlinking caverns.

"They're pretty technical caves," McEachern said. "A lot of vertical work, a lot of climbing. It's cold, wet, easy to get hypothermic."

He wandered there, deep beneath the scenic Bob, among rock painted crimson with iron -- the ferrous red running through Blood Cave's underground mineral veins -- among delicate soda straws and massive sheets of slow-motion flow stone.

Science calls these remarkable and colorful formations speleothems, but cavers know them better as gypsum flower, angel's hair, cave pearl. Stone poetry.

Such otherworldly beauty is one reason McEachern goes deep. Another is the adventure, and another the hard-wired explorer's drive to chart the uncharted.

That's why we went, too, in our small way, and also because it's good to be 8 years old and exploring new places on a fall afternoon. It feels right, in a scary kind of way, to have new adventures in unknown places.

So we ducked into the cave north of Polebridge and the sandy ground sloped away steeply, pinching into a flat-bellied wiggle before opening onto sculpted hallways of living stone.

This, we knew now, is a sport for the long and the lean.

Farther on, once the world was lost to deep dark, an underground stream crossed our path, rippling shallow to the right, sliding deep and dark to the left. We'd read about this, and so splashed through the shallows, over brilliant river rock shining red and green and even bright blue beneath our headlamps, crouching and crawling and slithering and then standing tall again in great rocky rooms.

We kept track of the forks with pink surveyor's ribbon, which could be collected on the way home without a trace, because rocks look a lot alike to me. Even McEachern's been turned around down under, circling and circling and wondering, "Did I see that before I got lost, or after I got lost?"

It was for all of us, in many tight spots, an exercise in pushing beyond boundaries, those walls we build for ourselves without even knowing it. We tore them down, kept moving, squeezed through, whistled in the dark.

And when the icy water flowed too deep, we dug deep into a backpack for our tiny inflatable raft and rode the underground river, one at a time, pushing hand over hand against the ceiling to travel silently upstream. It was a wonder.

Not far from where we explored is another cave, a cave of pools swimming thick with isopods and planaria and olilgochaete worms, even an aquatic amphipod brand new to science. But all we found was a packrat, nesting near the entrance in a dry little grotto all his own.

"They're common," McEachern admitted, "and they're prone to mischief."

By which he means they'll chew your rope through, which is more than enough mischief if you're at the bottom of Virgil the Turtle, looking up at the wide-open welcome of a star-studded Big Sky.

But the packrat posed us no problems, and when we emerged it was as if to a different world, which is to say a world of light and life, of warmth and wind. It was also a world that now looked a bit altered, because we had seen it from beneath. We knew what was underfoot, and that changed things.

I had often told my 8-year-old, at bedtime, not to worry about the dark because without it we wouldn't be able to see the stars. And now I knew it really was true -- without the dark, the light does not shine as brightly. We need them both.

Because without the dark, and a glimmer of light, too, you'll never see that shooting star in time to duck, before it smacks you in the forehead.