What is red mercury?

Three men have been cleared of trying to procure the raw ingredients for a "dirty bomb" which the prosecution claimed could have devastated a British city if it fell into the hands of terrorists. But mystery surrounds the material at the centre of the plot. So what exactly is red mercury?

The most bizarre aspect of the trial of Abdurahman Kanyare and his two co-defendants was the fact that no-one in the court could be certain whether the terrifying substance on which the entire prosecution case was based actually existed.

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. [One theory: It is ] a military codeword for a new nuclear material, probably manufactured in Russia. - bbc