“fireproof safe in the basement.”
Basements, also known to fire departments as “Built In Swimming Pools”. They tend to get filled up by the fire department when they pour water all over your burning house. You might want to also ensure the backups are not only fireproof but water proof as well.
Basement=two car garage plus workshop. Those garage doors are not exactly watertight but I see your point. Of course the backups are to DVD disks so water is not an issue.