At a certain moment in the late 1960s, the lava lamp came to symbolize all things countercultural and psychedelic—although, as you might expect, those who basked in its lurid glow sometimes had trouble recalling exactly why. It’s like asking, “Why did we like Jackson Pollock?” says Wavy Gravy, the longtime peace activist and Grateful Dead sidekick. “Because it was amazing! It causes synapses in your brain to loosen up.”

The mesmerizing light fixture, which turns 50 this year, has risen and sunk and shifted its shape in the cultural consciousness for decades. The lamp was invented by Edward Craven Walker, a British accountant whose other claim to fame was making underwater nudist films. He was passing the time in a pub when he noticed a homemade egg timer crafted from a cocktail shaker filled with alien-looking liquids bubbling on a stove top.

Determined to perfect the design, and to install a light bulb as the heat source, he settled on a bottle used for Orange Squash, “a revolting drink we had in England growing up,” according to Cressida Granger, who today owns Mathmos, the successor to Craven Walker’s original company, Crestworth Ltd. Craven Walker’s lamp paired two mutually insoluble liquids: one water-based, the other wax-based. The exact recipe is a proprietary secret, but a key ingredient is the solvent carbon tetrachloride, which adds weight to the otherwise buoyant wax. The heat source at the bottom of the lamp liquefies the waxy blob. As it expands, its density decreases and it rises to the top—where it cools, congeals and begins to sink back down. By the end of the decade, Craven Walker’s company was manufacturing millions of “Astro Lamps,” as he called them, per year. In 1965, he sold the U.S. manufacturing rights to a company called Lava Lite.

Craven Walker didn’t envision the lamps as paragons of grooviness. “They weren’t marketed like that—they were almost staid,” Granger says. Indeed, an ad in a 1968 edition of the American Bar Association Journal touted the “executive” model—mounted on a walnut base alongside a ballpoint pen.

As light fixtures go, the lamps are peculiar in that they don’t cast much light. They appeal to people at ease in darkness. Rather than illumination, the purpose of lava lamps is “to create mood,” says Stephen Horner, who teaches lighting design at the Pratt Institute in Brooklyn, New York. The fixture’s dynamic interior, he says, harkens back to one of our most ancient forms of amusement: the flickering hearth. Yet the sleek, rocket-like exterior was perfectly pitched to the space age.

Tastes changed and the lava lamp craze cooled by the late 1970s. In 1989, when Granger met the then-septuagenarian Craven Walker at a nudist camp (both were clothed, at her request) to discuss her interest in buying Crestworth, it was manufacturing only about 1,000 lights per year. But amid the Austin Powers-fueled nostalgia of the 1990s, the public again warmed to the lamps, and in 2000 Mathmos sold some 800,000. The U.S.-based Lava Lite supplies millions per year to retailers such as Target and Wal-Mart.

Mathmos has also dabbled in screen savers, LED lights and other modern novelties, but the lava lamp has proven its staying power: The company still takes orders from original 1960s-era owners who need replacement bulbs.