CRAVEN WALKER HAS A THEORY about people who don't like lava lamps: "They're frightened of sex." As the 77-year-old, Singapore-born inventor of the kitschy relics sees it, each oily undulation "starts from nothing, grows possibly a little bit feminine, then a little bit masculine, then breaks up and has children. It's a sexy thing."

And a half-century-long obsession of Walker's. He came across the lamp's prototype, a "contraption made out of a cocktail shaker, old tins and things," he says, during a visit to a Hampshire, England, pub shortly after he left the Royal Air Force at the end of World War II. Purchasing the liquid-filled fixture, he then learned its creator, whom he knew only as a Mr. Dunnett, had died. Determined to make his own version, Walker formed the Crest-worth Company in Dorset, England, and over the next 15 years, in between running an international house-swap agency and making films about his other passion, an environmentalist's form of nudism called naturism, tried to build a better lava lamp. The first models hit British stores in the early '60s, and some shop owners didn't appreciate the lamp ("They'd say, 'Take it away, it's disgusting' "). But by the time psychedelia took hold in the mid-'60s—and Walker had perfected a still-secret lava recipe of oil, wax and "other solids"—demand for the sensuous light sources reached fad proportions. By 1990, when his company was bought by London '60s buffs Cressida Granger and David Mulley, Walker had sold some 7 million of his creations.

Although lava mania cooled after the '60s, a growing demand for hippie icons in the '90s started a new lava wave. These days, the company ships 400,000 lamps a year to shops around the world, where customers pay up to $160 for the freak-out fixtures.

Now married to his fourth wife, Sue, 44, a former bar manager, and still working as a technical adviser to his old company, Walker sees other benefits to lava gazing. "You can avoid going on drugs," he says with a smile. "If you have a lava lamp, you don't need them."