It was named for a tiny Pacific atoll where the U.S. once tested atomic bombs, and indeed its eye-popping debut once sent shock waves around the world. Later, it became the calling card for such Hollywood sexpots as Marilyn Monroe, Ursula Andress and Raquel Welch, and the meal ticket for model sun goddesses like Cheryl Tiegs and Christie Brinkley. No doubt it has saved marriages (and destroyed others), started affairs and finished puberties. It does more for men's hearts than an hour of jogging. And to be so tiny.

Say happy birthday to the bikini.

It was 40 years ago this month in Paris when French civil engineer and designer Louis Reard unveiled his bathing brainchild, shrewdly named for the Bikini Atoll, where explosions had been set off four days earlier. The first bikini was three measly pieces of cotton, measuring 129 square inches and held together by two strings and a prayer. So risqué was Reard's invention that no model would show herself in one. Finally, Reard hired striptease dancer Michèle Bernadini to pose. She did. Briefly.

Forty years later, the bikini—once denounced as pure fad—accounts for nearly 20 percent of annual swimwear sales, decorates beaches from Far Rockaway to Rio and is once again enjoying a sales spurt. Says New York fashion analyst Bernard Ozer: "Life begins at 40, and the bikini is no exception."

The world first headed for Bikini Beach with reluctance. After its dramatic debut, the gravity-defying suit was banned in Italy, Spain, Portugal and Belgium. Its reception on these shores was equally chilly: Hollywood's Esther Williams flatly refused to wear one, and Sears Roebuck airbrushed out the navel of a bikini-clad catalog model. Swimsuit designer Monika Tilley remembers when "people in bikinis were carted off Jones Beach for indecent exposure."

It was a decade later—when voluptuous French sex kitten Brigitte Bardot tripped across the sands at the 1956 Cannes Film Festival in a two-piece—that the bikini began to be taken seriously in the swimsuit market. Its mainstream acceptance was validated in the late 1960s when the word bikini entered Webster's dictionary without the capital B.

Over the years, the bikini has taken some faddish turns. In 1964, Rudi Gernreich developed the "monokini," a bottoms-only suit, with suspenders to cover only the nipples. Like the Edsel, it never quite caught on. A year later, Reard introduced the microscopic "sexykini," hoping to boost sagging sales figures. That bombed, too. These days much ado about nothing is being made over the daring fio dental (dental floss) number, barely visible on the beaches of Brazil. Domestically, bikinis are cut thigh-high and topped by turtle-neck halters and made from body-clinging fabrics such as spandex and Lycra to suit aerobics-and-iron-wrought bodies. "My philosophy is if you have a good body, show it off," declares bikini designer Ellen Ann Dobrovir.

Give credit for that attitude—and the bikini's long life—to the sexual revolution. As designer Anne Cole, the elder stateswoman of the pack, puts it: "We were taught nice girls didn't wear bikinis. But nice girls went out in the '60s."