Immutable

In blockchain systems, immutability comes from how blocks are chained together, not from a rule that simply forbids editing data. Each block stores a cryptographic hash of the block before it, so changing a past transaction would alter that hash, break every link after it, and force an attacker to re-mine all later blocks faster than the honest network extends the real chain. That is what makes history practically, rather than absolutely, unchangeable.

Immutability is a matter of cost, not a law of physics. On proof-of-work networks, anyone controlling more than half the total hashing power can attempt a 51% attack, secretly mining a longer private chain and forcing nodes to swap to it. Ethereum Classic was hit this way in 2019 and again across three attacks in August 2020, reversing millions of dollars in transactions; Monero suffered a comparably deep reorganization in 2025. Smaller networks with thin hash rate or concentrated stake are far more exposed than Bitcoin or Ethereum, where rewriting settled history is prohibitively expensive.

Rollbacks can also happen by community agreement rather than attack. After the 2016 DAO hack drained roughly $60 million in ether, the Ethereum community hard-forked the chain to rewind its state to before the theft. Participants who rejected that override kept mining the original, unaltered history, producing a permanent chain split still known today as Ethereum Classic, proof that immutability only holds as long as participants choose to uphold it.

This same guarantee is why deployed smart contracts are treated as final, since their code and history generally cannot be edited afterward. The more confirmations that stack on a transaction, the closer it gets to true finality, as each new block makes reversal exponentially harder.