Rumours that soviet nuclear experts had produced a mysterious explosive material with unimaginable destructive power first circulated in the 1970s, and despite several official investigations and subsequent denials the story refuses to die. The near-mythical compound cropped up again on sunday, when the news of the world claimed it had foiled a terrorist plot to buy red mercury as material for a dirty bomb. Depending on who you believe, red mercury is either an elaborate hoax, a codename for nuclear material smuggled through the former iron curtain, or a terrifying new trigger for a handheld hydrogen bomb. What it isn't, according to the speculation and hearsay that makes up the scientific literature on the subject, is any use for a dirty bomb (one that scatters radioactive material) .