This is a more technical post than most which appear here, on the apparently arcane point of whether blockchains actually are (or even should be) as immutable as is sometimes claimed for them. That matters to those interested in the use of blockchains, rather than their technology, for two reasons. The first is that although the distributed design of a blockchain – particularly a public blockchain such as bitcoin – makes it hard to compromise, hard is not the same as impossible, and understanding where motive and opportunity might overlap is important to decisions about how and when it is sensible to use them. The second is that the pattern of interests and opportunities may be different for large institutions such as government. Public blockchains are kept honest by large, computationally intensive processes which, by design, are expensive and inefficient, but which allow participants in exchanges to be trusted without needing to prove that they are trustworthy. If other forms of trust are available, the overheads of bitcoin-like blockchains can be avoided, bringing a slightly different mix of risks and opportunities.
The article also contains a neat and succinct description of how blockchain actually works (though it might not be the ideal starting point for people completely new to the subject).