Beneath South Dakota's Black Hills

More than a century ago, an army of fortune-hunters descended upon the Black Hills of South Dakota in search of gold. Many did find gold, but they also happened on another underground treasure: one of the most extensive and spectacular limestone cave systems in the world.

A hidden world of hundreds of miles of craggy, twisting passageways - created over eons and adorned with intricate and bizarre crystal formations - snakes below the prairies and low granite mountains of western South Dakota. Countless mineshafts also burrow into the hillsides, monuments to man's eternal lust for gold.

Most visitors to the area come to see the four Presidents carved into Mount Rushmore or the surreal topography of the Badlands, bypassing Wind, Jewel and the many other caves and mines. Most of the caves and mines - including the largest operating gold mine in America - are open to the public from late spring through mid-October, and an outstanding geology museum in Rapid City offers a year-round introduction to the area's underground wonders.

The most dramatic subterranean environment in the Black Hills is Jewel Cave. It was discovered in 1900 by two prospectors who felt a mysteriously strong breeze blowing from a tiny mountainside hole. They dynamited their way inside, only to discover tunnels branching off in every direction. Instead of gold, they found walls filled with sparkling crystals. Tourists began to flock to the cave to see its eerie formations, and in 1908, Jewel Cave was proclaimed a national monument. The National Park Service built trails and stairs, and, in 1972, opened a 230-foot elevator shaft making the cave more accessible than ever. Seventy-four miles of rocky, underground corridors have been mapped to date.

Most geologists believe that the Black Hills caves were formed about 50 million years ago by underground water dissolving layers of limestone. The water oozed through cracks and openings, gradually eating away at the soft sedimentary rock to create a network of narrow tunnels and more spacious rooms.

The slow but persistent flow of water also brought dissolved minerals - particularly calcium carbonate, a component of limestone - into the caverns, depositing them on cave walls, ceilings and floors to form ornate, multicolored crystals. These so-called speleotherms come in every size and shape: Some appear as glistening, gemlike beds of translucent nailhead or dogtooth spar. Other, icicle-like stalactites and dripstone, dangle from the ceilings, while rippled curtains of molasses-colored flowstone hang from the walls.

Delicate calcite crystals known as helictites seem to defy gravity, twisting sideways and upwards, and tiny, white bubbles of ''cave popcorn'' alternate with fine, hairlike needles called frostwork. Tangled blades of calcite that intersect to form erratically shaped chambers known as boxwork cling to cave ceilings. Hollow stalagmites look like dripping candles.

It is always a damp and chilly 53 degrees in the cave, and the moisture attests to the fact that Jewel is a living cave. Crystals are still forming, albeit very, very slowly. Stalactites are believed to grow an inch a century. The cave is also silent and dark, except for the lights planted by the park service and the tour guide's commentary. (Inevitably, the guide will flick a switch midway through the tour and announce that this is the meaning of ''total darkness.'') The park service offers several tours, including the most popular scenic tour, accessible by elevator; a historic candlelight tour beginning at the cave's natural entrance a mile from the visitor center, and a much more strenuous spelunking expedition for adventurous contortionists willing to crawl for four hours through jagged-edged holes.

Jewel Cave's park service companion, Wind Cave, is best known for its boxwork formations and its scenic, 44-square-mile above-ground national park. In 1903, this became America's first national park dedicated to a cave, but today the caverns are neglected by many eager tourists hastening to get to Mount Rushmore, 20 miles to the north. Motorists, however, are treated to herds of placidly roaming buffalo and comically screeching prairie dogs along Routes 385 and 87, the main streets of Wind Cave National Park and adjacent Custer State Park. Indeed, the grasslands and low hills of both parks are home to an abundance of other wildlife, including elk, pronghorn antelope, mule deer and coyotes.

Wind Cave's entrance, about 35 miles southeast of Jewel Cave, was also discovered as a result of an unexplained gust of wind. Many of its wonders are made easily accessible by an elevator, staircases and paved walkways. The half-mile tour takes visitors through an array of evocatively named rooms such as the Garden of Eden and the Blue Grotto. Although every cave in the Black Hills seems to have its temple and assembly, cave explorers have concocted more imaginative appellations, such as Max Head Room, named by a ranger, Darren Ressler.

''One of the privileges of exploring a cave is naming it,'' he explained.

Most Black Hills caves are embedded about 300 to 700 feet beneath the surface in a layer of rock called Pahasapa Limestone, named for the Sioux words meaning black hills. It was deposited on the floor of a shallow, prehistoric sea that covered much of western North America about 350 million years ago. Perhaps 280 million years later, about the time the dinosaurs became extinct, the Black Hills were thrust upward and the process of cave-formation began, according to Phil Bjork, director of Rapid City's Museum of Geology.

A cave might seem like an odd piece of real estate to own, but about a half-dozen entrepreneurs operate caves that offer public tours. Wonderland Cave, southeast of Deadwood via Route 385, is one of the more amply endowed caverns. Dripping stalactites, dainty helictites, flowstone and popcorn can be seen in abundance. A 40-foot stalactite fence and a reflecting pool beneath a low ceiling of immaculate white crystals are other noteworthy features.

Rushmore Cave is another cave whose discovery dates back to gold-rush days. Just off Route 40, near Keystone and Mount Rushmore, this cave includes a room almost as long as a football field filled with tens of thousands of cream-colored stalactites, and Rushmore also has many fossils that are visible in its walls.

Black Hills Caverns are just four miles west of Rapid City on Route 44, and include extensive displays of flowstone, dogtooth spar, boxwork and tiny cave pearls. This cave has a Chapel Under the Hills, where weddings have been performed amid the baroque natural formations. A 75-foot-high vault, a bottomless pit that drops to the water table, and a Moonshine Room, where bootleggers hid in the 1890's, are also featured.

Other private caves with tours include Sitting Bull Crystal Caverns, about 10 miles south of Rapid City on Route 16, which is noted for its dogtooth-spar crystals and fossils of prehistoric sea life. Three miles west of Rapid City is Crystal Cave Park, which has a variety of crystal formations. Stagebarn Crystal Cave is a small, still-forming cavern 10 miles north of Rapid City that also offers tours. Even waterfalls are found underground in the Black Hills. Thunderhead Falls cascades within in an old gold mine, 10 miles west of Rapid City on Route 44.

Uplift, erosion and other eons-long natural processes may have created the most grand subterranean world, but human avarice also played a part in gouging out the gentle mountains of western South Dakota. After Gen. George Custer's 1874 expedition discovered gold in the Black Hills, thousands of prospectors, gunslingers and assorted pioneers transformed the amorphous dominions of the Sioux Nation into the mythic heart of the Wild West.

The Homestake Mine, in Lead, has operated continuously since 1876, a year before George Hearst purchased the claim for $77,000. Homestake, which is the largest, deepest and most productive gold mine in North America, has yielded more than $1 billion in gold over the years. Although the mine's 50 underground levels, are off limits to the public, guided surface-level tours show how ore is hoisted, crushed and milled. The Black Hills Mining Museum, on Route 85 and Main Street in Lead, tells the story of South Dakota's gold rush and gives a taste of mining life. The nearby town of Deadwood was reputedly the toughest city in the world during the days when gold was king and the likes of Wild Bill Hickok and Calamity Jane walked the main street of Deadwood Gulch. Today, myth and reality are woven together in this town of swinging saloon doors, painted-over bordellos and a ghostly, below-ground Chinatown.

Gold mania and railroad contruction brought hundreds of Chinese immigrants to Deadwood in the 1870's, and many are said to have used a maze of underground walkways as hideouts and opium dens. Whatever the truth may be, these dark, dank and black-lighted tunnels - replete with opium vials and pipes - are now called the Chinese Museum Tunnel and are open to the public.

The main drift (tunnel) of the now-defunct Broken Boot Gold Mine runs 840 feet into the hillside to giant stopes where gold ore was once mined. Today, the mine offers underground tours of the craggy, hard-rock drifts where, long ago, miners picked away at the hillside. The mine is a mile west of Deadwood on Route 14A.

Given the profusion of geological wonders in the Black Hills, it should come as no surprise that the area has one of the best small museums devoted to minerals, fossils and nature's sculpture. The Museum of Geology at the South Dakota School of Mines and Technology on the east side of Rapid City, on Route 79, has excellent displays of beautiful and rare crystals found in local caves. Its giant plesiosaur skeleton looks like a mock-up for the Loch Ness monster, but is actually testimony to the wealth of dinosaur fossils found nearby. Mammoths, sabertooths and a pregnant plant-eating creature known as an oreodont are among the other outstanding fossils. A GUIDE TO THE BLACK HILLS AREA When to Go

Jewel Cave National Monument, Box 60AA, Custer, S.D. 57730; 605-673-2288. Scenic tours, 8:30 A.M. to 6 P.M.; $3, $1 for children. Historic tours, 10 A.M., 1 and 3:30 P.M.; $3, $1 for children. Spelunking tour, 12:30 P.M.; $5. Some tours are offered year-round.

Wind Cave National Park, Hot Springs, S.D. 57747; 605-745-4600. Half-mile and mile tours, 8 A.M. to 6 P.M.; $3 and $4, $1 for children. Candlelight tour, 11 A.M. and 2 P.M.; $4, $1 for children. Spelunking tour, 1 P.M.; $5. Campfire programs, 9 P.M. Some tours are offered year-round. Wonderland Cave, Post Office Box 83, Nemo, S.D. 57759; 605-578-1728. Tours, mid-May to mid-October; $5, $2.50 for children.

Rushmore Cave, H.C. 89 Box 21, Keystone, S.D. 57751; 605-255-4467. Tours, May to October; $5, $2.50 for children.

Black Hills Caverns, Route 8, Box 570, Rapid City, S.D. 57702; 605-343-0542. Tours offered mid-May to Oct. 1; $4.75. Spelunking tours by appointment.

Sitting Bull Crystal Caverns, Post Office Box 1760, Rapid City, S.D. 57709; 605-342-8008. Tours, April to mid-October; $4.50, $2.25 for children.

Crystal Cave Park, Route 8, Box 364, Rapid City, S.D. 57702; 605-342-8008. Tours, mid-May to mid-October; $4.75, $2.50 for children.

Stage Barn Crystal Cave, Piedmont Route, Box 442, Piedmont, S.D. 57769; 605-787-4505. Tours, April to mid-October; $4, $2 for children.

Homestake Gold Mine, Box 887, Lead, S.D. 57754; 605-584-3110. Tours, weekdays from 8 A.M. to 5 P.M., May 1 to Sept. 30; $2.50, $1.50 for children.

Broken Boot Gold Mine, Route 14A, Deadwood, S.D. 57732; 605-578-9997. Tours, mid-May to Oct. 1; $3, $2 for children.

Big Thunder Gold Mine, Box 706, Keystone, S.D. 57751; 605-666-4847. Tours, mid-April to mid-October; $4.50, $2.25 for children.

Black Hills Mining Museum, Route 85 and Main Street, Lead, S.D. 57754; 605-584-1605. Open Memorial Day to September. Hours, Mondays through Fridays, 9 A.M. to 5 P.M.; Saturdays, 10 A.M. to 5 P.M., and Sundays, 11 A.M. to 5 P.M.; $2, $1 for children.

Chinese Museum Tunnel, 665 1/2 Main Street, Deadwood, S.D. 57732. Hours, 9 A.M. to 9 P.M., May to September; admission, 50 cents.

Museum of Geology, South Dakota School of Mines and Technology, 501 St. Joseph Street, Rapid City, S.D. 57701. Hours, Mondays through Saturdays from 8 A.M. to 6 P.M. and Sundays from 2 to 8 P.M., June 1 to Sept. 1; Mondays through Fridays, 8 A.M. to 5 P.M. and Saturdays 8 A.M. to noon, Sept. 1 to June 1; admission is free.

For information on supervised expeditions to a dinosaur quarry near Zeona, S.D., call 605-394-2467. Getting There

Continental, United and Northwest airlines fly from New York to Rapid City, S.D. Many other airlines fly to Denver, which is a long, but scenic 400-mile drive away from western South Dakota. Accommodations

Some places to stay are only open from about May through September, so it is wise to call ahead. A comprehensive guide to accommodations can be obtained from the South Dakota Department of Tourism, 711 Wells Avenue, Capitol Lake Plaza, Pierre, S.D. 57501; 800-843-8000.

Rapid City has several good hotels, including the elegantly restored Alex Johnson (523 Sixth Street, Rapid City, S.D. 57701; 605-342-1210 or 800-843-8800, extension 25), where double rooms are $61 to $71, and the Hilton (445 Mount Rushmore Road, Rapid City, S.D. 57701; 605-348-8300), with doubles at $65.

Comfortable motels in other Black Hills towns include the Bavarian Inn (Custer, S.D. 57730; 605-673-2802), with double rooms from $56 to $62; the Best Western (137 Charles Street, Deadwood, S.D. 57732; 605-578-3241), whose doubles are $53, and the Kelly Inn (Box 654, Keystone, S.D. 57751; 605-666-4483), with doubles at $49.

Among the rustic lodges with secluded cabins in Custer State Park are the State Game Lodge, famous as Calvin Coolidge's summer White House (H.C.R. 3, Box 74, Custer, S.D. 57730; 605-255-4541). Rates range from $35 to $60.

Some of the nicest of the area's 25,000 campsites can be found at Elk Mountain Campground in Wind Cave National Park (Hot Springs, S.D. 57747; 605-745-4600). Restaurants

Dining in the Black Hills need not mean only beef or buffalo, although both can be had in abundance. The Sluice restaurant in Spearfish (605-642-5500) is renowned for its 30-page menu. You can get everything from trout with almonds and roast buffalo to miner's stew and a build-your-own-sandwich bar are available, with prices ranging from $2.50 to $19.95.

Rapid City restaurants include the Landmark at the Hotel Alex Johnson and the Firehouse (610 Main Street; 605-348-1915), both of which feature prime rib and steak, with dinners about $10 to $15 at both. The Deadwood-Lead area boasts several good and colorful places to eat - all of which are strong on beef and seafood, with dinners about $6 to $15. Among them are the Grubstake (305 West Main Street, Lead; 605-584-1984) Dave's Supper Club (Route 14A, Central City; 605-578-9942), and the 1903 Franklin Hotel Dining Room (700 Main Street, Deadwood; 605-578-2241). - A. L. Y.