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Google says four successive lightning strikes on one of its cloud infrastructure facilities in Europe last week permanently wiped out some customers' data stored on disks. Google called the incident "exceptional" and apologized to those affected.

In an online statement Wednesday, Google said the series of unlucky strikes happened on Aug. 13 at its data center in Belgium. The lightning briefly knocked out power to systems that host customers' data and allow clients to run virtual machines "in the cloud."

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"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," Google said. "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 percent of total disk storage space — the data was unrecoverable and permanently lost.

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Google said it is working on hardware and software improvements to "maximize the reliability" of the data center.

"Google takes availability very seriously, and the durability of storage is our highest priority. We apologize to all our customers who were affected by this exceptional incident," the company said.

Updated 8:09 p.m. ET Wednesday: