The chaos caused over the late May Bank Holiday by the IT failing at Gatwick & Heathrow was caused by a power outage and then a subsequent serge.
The cause of the initial outage has not been revealed however it is possible that a power cable could have been severed as part of ongoing development works at Heathrow.
BA at Heathrow were however instrumental in the drafting of PAS 128 and take the hazards from buried services very seriously. The congestion of services at Airports is significant, as shown by the below extract.
Extract of drawing from Airport Survey
GEOTEC Surveys have carried out Underground Mapping, both Airside and Landside, as part of major airport development projects, at London City, London Luton, Heathrow and Stansted in the recent past.