Professor Lord Martin Rees has revealed the worst case scenario" for particle accelerators - and they could mean the end of Earth as we know it. He warns that if things went wrong, they could result in a black hole being formed, or the Earth being turned into a "hyperdense sphere". Particle accelerators like the Large Hadron Collider shoot particles at incredibly high speeds, smash them together, and observe the fallout. While they have led to massive breakthroughs ... they also carry a high risk, Rees says in his new book On The Future: Prospects for Humanity. "Maybe a black hole could form," he writes. "The second scary possibility is that the quarks would reassemble themselves into compressed objects called strangelets. Under some hypotheses a strangelet could, by contagion, convert anything else it encounters into a new form of matter, transforming the entire earth. Many of us are inclined to dismiss these risks as science fiction, but give the stakes they could not be ignored." Cern writes on their website "The LHC Safety Assessment Group (LSAG) reaffirms and extends the conclusions of the 2003 report that LHC collisions present no danger and that there are no reasons for concern. Whatever the LHC will do, nature has already done many times over during the lifetime of the Earth and other astronomical bodies."