In the darkness, we know our guide by the tongue of flame on his hatbrim. In front of our group, two boys of 4 and 5 pat over the muddy floor in identical white boots. While some of us shine flashlights on the stony ceiling and walls, and on the darkened path ahead, Chris and Brian shine theirs on puddles that ripple with water dropping from above.

We are under the Lincoln Memorial.

Wearing a miner's carbide lamp, Jim Burgess is leading 15 people in National Park Service's twice-weekly journey into another dimension. Everything has an underside - even a national monument. Man made this underground cavern, but nature's doing the decorating. We're here to see the evidence of both.

"If you're a beer-can collector, you could have a field day here," Burgess is saying. "But we leave them for archaeological purposes. Maintenance workers had a picnic down here. Be careful where you're walking. Some of that trash might be valuable some day."

We approach a roped-off area. Within it, a six-pack or more of Schlitz pop-top cans are half-buried in earth. Several visitors giggle. We are informed the ropes keep people like us out of a crumnling hole. We quickly move on.

Standing between support columns under the 175-ton statue, my 11-year-old nudges me. "A shoe . . . It's disintergrated!" Then, "Look! Part of a drill!"

This is like visiting someone's living room when no one's been there for 56 years, cleared out the cobwebs or old cigarette butts, or fixed it up to charge admission.

The workers drew on the walls here: Immortalized in charcoal is a pipe-smoking and rotund person, who could be the foreman, hands comfortably tucked in his pockets. Burgess tells us the workers had eight years, from 1914 to 1922, to play while building the memorial. Whoever the artists, they scrawled caricatures of (probably) Mutt and Jeff and President Wilson. "We got a horse there," Burgess ventures. "Donkey," several adult visitors correct. "Donkey, okay."

It's quiet down here, except for drip, drip, drip of water seeping through cracks overhead. Rainwater combines with carbon dioxide in the air to make carbonic acid, monument. We see the first underground evidence of this chemical weathering: a limestone stalagmite. One of the little explorers is concerned: "It's gonna grow up through the ceiling?"

Ranger Burgess leads us into what he calls his underground obstacle course - the gallery of support beams under the memorial's steps and front plaza.

We flash our lights into recesses of the mammoth chamber and illuminate row upon row of infinitely receding columns and beams. To see the stalactites and stalagmites, we follow Burgess. Clutching our flashlights, we clamber seat first or jump feet first from one concrete support beam to another. We cover 50 or 60 of them. No one, trips very much.

Halfway through the course, amusement at our various degrees of clumsiness has changed to grumbling: Is he kidding? But no one would stand alone to await the group's return: It's too dark. Naturally, the little ones love it. Their parents have been passing them over the beams like small bags of cement. Chris says, "Daddy, let me try, okay?"

The kind of stalactites we're seeing look like stone icicles. Spelunkers aptly name this sort of stalactite a soda straw because it's hollow inside. A few of the thousands here have grown to be six feet long.

Burgess says there could be formations like this under other buildings. "It's very similar to this under the Jefferson Memorial, but since it's not quite as old, the formations aren't as good.But they're growing."

Any more questions?" he wants to know as we start our return trek.

"How're you doin', mom?" a young and energetic woman asks.

"What a way to spend a wedding anniversary," replies her mother.

"This is the only cave you can visit in Washington," says Burgess.

"Yeah," says mom.

"You can even hear the tourists lumbering up the stairs!" says the cheerful ranger.

". . . a dream of a lifetime . . . "

Like Burgess, my son is enthralled: "I can't wait 'til a hundred years from now when it looks like a real cave!"

We find our way out into the night through a door at the base of the memorial. When we met by the statue it was raining and we had Lincoln to ourselves. Now, an hour or so later, a bit muddy and tired, we see crowds, class trips descending the monutment stairs in a cascade of full skirts. They wonder about our flashlight and old clothes.

Chris and Brian have ambled out to the edge of the plaza. "We went all the way under to here," Chris marvels. "Went all the way under to here . . . " echoes his brother. Chris is pointing his flashlight at everything, including the familiar illuninated obelisk across the reflecting pool. "I can see the Washington Momument with my flashlight. I can see the spots on it."

"Oh no," Chris is told. "They use big floodlights to . . . oh, never mind."