The white clouds that you can sometimes see above the CERN sites come from one of two sources. They can come from the traditional cooling towers that are used to cool electrical power installations, for example, in which case the cloud is just water vapour. In other cases, they come from the LHC accelerator’s cooling system. In order to operate in a superconducting state, the LHC has to be cooled to -271°C. The first step in the cooling process involves introducing liquid nitrogen to cool the helium that will eventually circulate in the underground accelerator. During this process, the helium transfers its heat to the nitrogen, which thereby transforms from a liquid state to a gaseous state before being released into the atmosphere. The resultant cooling of the ambient air can momentarily create mist. This phenomenon is not at all dangerous since nitrogen is the main constituent of the air that we breathe, which is 78% nitrogen and 21% oxygen.