When many of the worlds best minds were working on the Manhattan Project prior to the first atomic bomb test (Trinity), it was seriously considered that the temperature of ignition could be so high that it would cause a spontaneous runaway reaction in other matter, such as a hydrogen hydrogen reaction in the worlds oceans. Eventually it was calculated that it wouldn't happen.
So this LHC is a case of building something and flicking the switch, but not really knowning what is going to happen. That has some element of danger.
Just imagine if supernovas aren't actually stars blowing up, but alien civilisations wiping themselves out after building LHC and turning them on?!
