How to inject carbon dioxide into hard candy, to make popping rocks?
Looks like it takes about 600psi CO2 to treat the liquid candy.
Patent search should reveal more detail: US patent 3012893 

Patent links are notorious for decaying over a short time frame.
If the second link is dead go http://www.uspto.gov/patents-application-process/search-patents or here to find find out how the inventors did it.
Patents before 1976, are in image format, and are hard to look through, sometimes Google is a better choice than USPTO on these.

See also US Patent 4289794 (1981)


  Gasified candy which produces a more pronounced popping sensation is prepared by maintaining a sugar melt at a temperature of below about 280° F. ... Such a candy is made by a process which comprises melting crystalline sugar, contacting such sugar with gas at a pressure of 50 to 1,000 psig for a time sufficient to permit incorporation in said sugar of 0.5 to 15 cm3 of gas per gram of sugar, maintaining the temperature of said sugar during said absorption above the solidification temperature of the melted sugar, and cooling said sugar under pressure to produce a solid amorphous sugar containing the gas. Upon the release of the pressure, the solid gasified candy fractures into granules of assorted sizes.


High temperature, high pressure. Even as a hard candy, the stuff has a short shelf life.
Baking soda/acid mixture candies are just Fizzies in disguise. Feel weird when they bubble in your mouth, but they do not explode like pop rocks.