Acording to Chris Summers of BBC News, "The five main theories are:
* That red mercury is a reference to cinnabar, a naturally-occurring mercuric sulphide. The red pigment derived from cinnabar is known as vermillion.
* That it is a reference to the alpha crystalline form of mercury iodide, which changes to a yellow colour at very high temperatures.
* That it is simply referring to any mercury compound originating from the former Soviet Union. The 'red' tag would simply be a legacy of the Cold War era.
* That it is a ballotechnic mercury compound which just happens to be red in colour. Ballotechnics are substances which react very energetically when subjected to shock compression at high pressure. They include mercury antimony oxide which, according to some reports, is a cherry red semi-liquid produced in Russian nuclear reactors. This theory contends that it is so explosive that a fusion reaction - a nuclear explosion - can be triggered even without fissionable material such as uranium.
* That it is a military codeword for a new nuclear material, probably manufactured in Russia."
"The prosecutor, Mark Ellison, admitted the police had no idea if there even was such a thing as red mercury - supposedly the main ingredient for a "dirty bomb" which could have devastated London." (Chris Summers of BBC News 25 July 2006)