Most of the security lies in the distributed nature of the blockchain.
If you want to secure your data, you encrypt. You make a backup copy. You make an offsite copy. Etcetera, etc.
The "blockchain" is a collection of parallel copies of an encrypted ledger. It's the equivalent of constantly backing up your data to hundreds of thousands - millions of offsite locations simultaneously.
ALL copies of the blockchain would have to be destroyed to truly destroy it. Only one copy could restore it. Two or more nodes, and transactions become possible. The internet isn't even necessary, only a network.
Blockchain is pretty robust, and about as good as it can get, in terms of recovering from a catastrophic disaster.