Some people have permanently lost access to their files as a result.

A number of disks damaged following the lightning strikes did, however, later became accessible.

Generally, data centres require more lightning protection than most other buildings.

While four successive strikes might sound highly unlikely, lightning does not need to repeatedly strike a building in exactly the same spot to cause additional damage.

Justin Gale, project manager for the lightning protection service Orion, said lightning could strike power or telecommunications cables connected to a building at a distance and still cause disruptions.

"The cabling alone can be struck anything up to a kilometre away, bring [the shock] back to the data centre and fuse everything that's in it," he said.

Unlucky strike

The Google Computer Engine (GCE) service allows Google's clients to store data and run virtual computers in the cloud. It's not known which clients were affected, or what type of data was lost.