A lightning storm in Belgium last Thursday hit Google’s St Ghislain data centre causing power loss and damage to disk storage, leaving some customers without access to data.

The facility was hit directly by four successive lightning strikes which immediately took down the centre’s operations from Thursday 13th until Monday 17th August, according to Google.

The damage caused to Google Compute Engine’s primary storage persistent disks housed in the data centre resulted in a four-day data outage for some European customers.

“At 09:19 PDT on Thursday 13 August 2015, four successive lightning strikes on the electrical systems of a European data center caused a brief loss of power to storage systems which host disk capacity for GCE instances in the europe-west1-b zone,” Google wrote in a blog post this week.

It continued: “Although automatic auxiliary systems restored power quickly, and the storage systems are designed with battery backup, some recently written data was located on storage systems which were more susceptible to power failure from extended or repeated battery drain.

“In almost all cases the data was successfully committed to stable storage, although manual intervention was required in order to restore the systems to their normal serving state. However, in a very few cases [less than 0.000001% of PD space in europe-west1-b], recent writes were unrecoverable, leading to permanent data loss on the Persistent Disk.”

Despite the uncontrollable nature of the incident, Google has accepted full responsibility for the blackout. However, it stressed to customers that “GCE instances and Persistent Disks within a zone exist in a single Google data center and are therefore unavoidably vulnerable to data center-scale disasters.”

To prevent similar disasters in the future, Google promises to upgrade its data centre storage hardware, increasing its resilience against power outages. According to the search giant, research is already underway to improve cache data retention and response procedures for system engineers.

Disaster recovery experts have warned that lightning does not always need to strike a facility directly to cause damage. Justin Gale of storm protection service Orion explained that cabling could be struck up to a kilometre away, and direct the shock “back to the data centre and fuse everything that’s in it.”