Tape backups! That's right, those cassette type things you used to backup your business and some homes with in the 90's. Google backs up their Gmail (and probably more) with this physical offline media.
Sit back for a moment and consider the logistics, the scale, planning, organization, etc. It is truly astonishing. This amazing feat paid off when Google lost some users' email across multiple backups in several data center locations.
Thank you Google for restoring our emails!