Pop Rocks were developed in 1956 by William Mitchell, a research scientist with General Foods. They were introduced to the market as a candy in 1975 (what took so long?) in selected areas, eventually being distributed nationwide. Kids enjoyed the candy that burst in the mouth, thanks to pockets of CO^2 locked within the crispy candy shells that burst in the mouth from the warmth.
Not long after the candy became popular, an urban legend began spreading that eating Pop Rocks had caused a boy's stomach to explode; he had apparently swallowed a whole packet of the stuff at once (not that there was ever much candy within a packet) and quickly chased it down with half a can of carbonated soda - and the resulting carbonated bubble had caused his death. (The boy in question was even thought to be known to all of us as Mikey, the reluctant kid from the old Life cereal commercials, played by actor Michael Gilchrist.)
Parents around the country forbade their children to buy the deadly confection, and harassed local stores to stop carrying it. The stuff was perfectly harmless, of course - it had been tested thoroughly before being marketed, and had been consumed by probably millions of kids - but rumor is more powerful than reality, and Pop Rocks sales plummeted. General Foods sent Mitchell, the candy's inventor, to speak at schools and various civic groups to dispel the rumor; they took out full-page ads in major newspapers and magazines, and mailed 50,000 letters to school principals around the country. But, alas, it did no good. Within a couple of years, the candy was taken off the market.
Kraft bought the rights in 1985 and re-marketed it as Action Candy, but sales never regained their previous status. Pop Rocks are available again today, however, distributed by Pop Rocks, Inc., and can be found in selected areas.