Large Hadron Collider
Concerns have been raised that performing collisions at previously unexplored energies might unleash new and disastrous phenomena. These include the production of micro black holes, and strangelets, potentially resulting in a doomsday scenario. Such issues were raised in connection with the RHIC accelerator, both in the media and in the scientific community; however, after detailed studies, scientists reached such conclusions as "beyond reasonable doubt, heavy-ion experiments at RHIC will not endanger our planet" and that there is "powerful empirical evidence against the possibility of dangerous strangelet production."
One argument against such fears is that collisions at these energies (and higher) have been happening in nature for billions of years apparently without hazardous effects, as ultra-high-energy cosmic rays impact Earth's atmosphere and other bodies in the universe. A concern against this cosmic-ray argument is that, if dangerous strangelets or micro black holes were created at LHC, a proportion would have less than the Earth's escape velocity (of 11.2 km/s), and therefore would be captured by the Earth's gravitational field, whereas those created by high-energy cosmic rays would leave the planet at high speed, due to the laws of conservation of momentum at relativistic speeds.
CERN's review concludes, after detailed analysis, that "there is no basis for any conceivable threat" from strangelets, black holes, or monopoles. However, the concern about the verity of Hawking radiation was not addressed, and another study was commissioned by CERN in 2007 for publication on CERN's web-site by the end of 2007.
The risk of a doomsday scenario was indicated by Sir Martin Rees, with respect to the RHIC, as being a one in fifty million chance, and by Professor Frank Close, with regards to (dangerous) strangelets, that 'the chance of this happening is like you winning the major prize on the lottery 3 weeks in succession; the problem is that people believe it is possible to win the lottery 3 weeks in succession'. Accurate assessments of these risks are impossible due to the present incomplete, or even hypothetically flawed, standard model of particle physics (see also a list of unsolved problems in physics).
Micro black holes
Although the Standard Model of particle physics predicts that LHC energies are far too low to create black holes, some extensions of the Standard Model posit the existence of extra spatial dimensions, in which it would be possible to create micro black holes at the LHC at a rate on the order of one per second. According to the standard calculations these are harmless because they would quickly decay by Hawking radiation. The concern from opposing civil society movements is that, among other disputed factors, Hawking radiation (which is still debated) is not yet an experimentally-tested or naturally observed phenomenon. Thus, the above mentioned opponents to LHC consider that micro black holes produced in a terrestrial laboratory might not decay as rapidly as calculated, or might even not be prone to decay and, if unable to rapidly evaporate, they could start interacting, grow larger and potentially be disastrous to Earth itself.
Strangelets are a hypothetical form of strange matter that contains roughly equal numbers of up, down, and strange quarks and are more stable than ordinary nuclei. If strangelets can actually exist, and if they were produced at LHC, they could conceivably initiate a runaway fusion process (reminiscent of the fictional ice-nine) in which all the nuclei in the planet were converted to strange matter, similar to a strange star.