content: Libra doesn't use proof of work (which does have the advantage that it isn't burning shed-loads of electricity in proof of work competitions). For at least the first five years, the blockchain is whatever authorised validator accounts say it is, secured by public key certificates (https://developers.libra.org/docs/the-libra-blockchain-paper). It would be perfectly feasible for them to agree to change the authorised version of Libra history. I can think of at least two temptations to do this: 1) It turns out that there is a bug in the infrastructure of Libra, or in some big contract secured by Libra Move smart contracts. Libra wants to revoke the contract / a court tells Libra to revoke the contract, or else. 2) Somebody posts something embarrassing to the blockchain (silly examples - the Declaration of Independence, or Lady Chatterley's Lover). and Libra/Facebook doesn't want this preserved for eternity. At this point Libra needs to decide whether it is going to be functionally identical to a Bitcoin-like Blockchain, or just playing at being a blockchain for the techno-hype but in fact no more a blockchain than any other big financial institution.