On 12 June 1973, staff at Geneva airport measured winds gusting up to 47 knots. A few kilometres away, where CERN lay right in the path of the storm, it was even worse. Torrential rain and hail devastated neighbouring vineyards and caused havoc inside the laboratory. Roads turned to rivers and buildings to swimming pools. The Proton Synchrotron was shut down as water in the basement rose to 1.80 metres.
Muddy water forced up from the drains made matters worse, but all staff mucked in to help. When the floods receded, everything had to be hosed down (treatment not usually recommended for electrical equipment!) and the repair gangs and cleaners got on with the slow job of recovery. See more pictures and a link to the CERN Courier article here.