Spelunk your way across the globe, from London's Cabinet War Rooms to Moscow's Mayakovskaya metro station

In junior high, some friends and I discovered a way into our town's rain sewers. With cigarette lighters to guide us, we crawled through dark concrete tubes to emerge in a chamber just high enough to stand in. Fifty feet below street level, we'd found the perfect adolescent hideaway. Tom Sawyer had a river; I had a sewer.

A growth spurt put an end to my crouched-over tunnel-crawling days, but, for me, the allure of the subterranean remained. Fortunately, history has obliged with hundreds of mysterious underground gems. So put away your sunscreen. That pesky hole in the ozone layer won't be a problem where we're going.

Cabinet War Rooms, London
A 21-room time capsule beneath the former Office of Works building in Westminster, the Cabinet War Rooms are a far better memorial to World War II than Europe's deserted battlefields. From here Churchill directed the British campaign. You can almost hear his shoe leather slapping against the linoleum floor. Wherever you look in this bunker-turned-museum-- at the thousands of pinholes on a world map marking torpedo strikes, at the BBC microphone that the prime minister used to address the nation -- the place simply resonates with the past.
Clive Steps, King Charles St.; 44-171/930-6961, fax 44-171/839-5897; open daily; $7.75.

De Beers Diamond Mines, Kimberley, South Africa
A lust for diamonds has led to some of the world's deepest holes, and no mines burrow deeper than in Kimberley. When you arrive at the complex, you're greeted by the Big Hole, a vast crater dug entirely with picks and shovels that yielded three tons of diamonds before it closed in 1914. After donning coveralls, boots, a hard hat, and ear protectors, you step into a cage elevator and plummet 3,000 feet into the black earth, listening to miners from the nearby DuToitspan Mine blast away with hydraulic drills and earthmovers. The largest stone ever found there, a colossal 616-carat gem, resides in the aboveground museum.
Tucker St., Kimberley; 27-531/839-4270, fax 27-531/839-4210; open Monday-Friday; reservations required; free.

San Clemente Basilica, Rome
No place better illustrates the eternal city's vertical time line than the stone steps of San Clemente. From the 12th-century Chapel of Santa Catarina, a staircase leads down one floor and 800 years to the barren lower basilica. Faded frescoes and some of the first known inscriptions in modern Italian depict the life of Saint Clement, the fourth Pope. A maze of passages leads to a temple with an altar to the Persian sun-god Mithras, worshiped by an all-male fertility cult. In the second century, Mithraism became popular among Roman aristocrats. The sound of rushing water from a hidden aqueduct makes you feel as if you're in Fellini's Satyricon.
Via San Giovanni, Laterano; 39-70/451-018; $1.

Mayakovskaya Metro Station, Moscow
Palaces for the people, Moscow's metro stations (which also served as World War II bomb shelters) are the Soviet era's greatest gift to the capital. Each is unique in design, from ornate Beaux-Arts grandeur to bold Constructivist utopia. Every visitor will have his or her favorite. Mine is Mayakovskaya, completed in 1938. Architect Alexei Dushkin won the Grand Prix at the Paris World's Fair for his stainless-steel columns supporting colorful ceiling mosaics, one of which depicts a plane dropping blue and yellow parachutists.
The Moscow subway system operates daily 5:30 a.m.-1 a.m.

Tomb of Seti I, Luxor, Egypt
Most visitors to the Valley of the Kings flock to Tutankhamen's grave, but I prefer the tomb of Seti I (1318-1304 b.c.). Its well-preserved bas-reliefs illustrate a passage from the Book of the Dead in which the god Ra navigates the evil underworld. Apep, a 52-foot snake symbolizing primeval chaos, slithers on several walls. The burial chamber's vaulted ceiling holds a dazzling map of the constellations, which makes it the only underground spot I know of where you can gaze at the stars.
Luxor Tourism 209-5/373-294; open daily; $6 for any three tombs, except for King Tutankhamen's and Queen Nefertari's.

Adalaj Vav, Gujarat, India
Ten miles north of the industrial city of Ahmadabad, the four-level Adalaj well, built in 1499 by Queen Rudabai, offers sanctuary from the arid plain's brutal heat. Vibrantly colored wall carvings and paintings-- depicting elephants, horses, erotic couplings, and the god Shiva-- line the shaft where steps lead down to a small pool of water. Hindu, Muslim, and Buddhist architectural styles are all in evidence, and the good queen furnished each level with an octagonal landing where workers and weary travelers once reclined.
Gujarat Tourism 91-79/658-9683; call for hours.

The Catacombs, Paris
Deep beneath the 14th Arrondissement, a sign reads, STOP! HERE IS THE EMPIRE OF DEATH. If that warning doesn't scare you, the grisly scene that follows just might. The skeletal remains of nearly 6 million people, including victims of the French Revolution, lie stacked in a psychedelic patchwork that trails off into dark, echoing corridors. Originally a series of Roman stone quarries, the Catacombs weren't associated with death until the city consolidated its festering and overcrowded graveyards here in 1785. When the Nazis occupied Paris, the Resistance set up headquarters in these dank passages. Not a bad hiding place considering that the seven-square-mile portion open to the public comprises only 1/700 of the original network.
1 Place Denfert-Rochereau; 33-1/43-22-47-63; closed Monday, call for hours; $5.50.

Ruakuri Cave, Waitomo, New Zealand
Wet suit, check. Helmet with headlamp, check. Inner tube, check. The three-hour "black-water rafting" trip down the Huhunui River in the Waitomo Caves is one of the more unusual adventures anywhere. Some 250 feet below the earth, headlamps provide the only light through a series of chambers dripping with stalactites and vaulted ceilings illuminated by glowworms. Be warned: At one point rafters must jump from a small waterfall into the darkness below. Guide Angus Stubbs insists they haven't lost a customer yet.
Book Ruakuri Cave tours at Black Water Café, Main Rd., Waitomo; 64-7/878-6219, fax 64-7/878-5190; three-hour tour $35.

Forestiere Underground Gardens, Fresno, California
Baldasare Forestiere came from Sicily to America in 1900 and found work building the subways in Boston and New York. After five years he'd saved enough money to start a vineyard. He purchased 70 acres of land in California, sight unseen. A thick layer of unfarmable soil, as hard as rock, awaited him. Beneath it, however, lay moist earth. So he dug . . . for the next 40 years. The result is a 65-room, igloo-like complex that includes a ballroom, an 800-foot tunnel big enough for cars, a library, a fishpond, and, most startling of all, orange and lemon trees whose branches sprout through holes drilled in the ceiling for light.
5021 W. Shaw Ave.; 559/271-0734; two tours daily; reservations recommended; $6.

Underground Tour, Seattle
Like Rome, Seattle is a town built on top of itself, but it is also built at sea level. In the 1880's Pioneer Square was not a pretty place. Toilets turned into fountains when the tide came in, and the constant rain washed out dirt streets. After a devastating fire in 1889, civic planners jumped at the opportunity to correct the inherent flaw. For three years they trucked in tons of dirt to raise the city by as much as 32 feet. Ladders led to the new, higher streets, but 17 people died when they forgot about the drop-off. Eventually, all the businesses on the lower level shut down. An eerie, sealed-off city remains, with faded signage, Old West storefronts, and ghostly hotel lobbies.
610 First Ave.; 888/608-6337 or 206/682-4646; 90-minute tours daily; $8.

David Knowles's latest novel, The Third Eye, will be published by Nan Talese/Doubleday in 2000.