Cybersecurity analysts call this Q-Day—the day someone builds a quantum computer that can crack the most widely used forms of encryption.

And then there’s Bitcoin. The cryptocurrency is exquisitely vulnerable to Q-Day. Because each block in the Bitcoin blockchain captures the data from the previous block, Bitcoin cannot be upgraded to post-quantum cryptography, according to Kapil Dhiman, CEO of Quranium, a post-quantum blockchain security company. “The only solution to that seems to be a hard fork—give birth to a new chain and the old chain dies.”But that would require a massive organizational effort. First, 51 percent of Bitcoin node operators would have to agree. Then everyone who holds bitcoin would have to manually move their funds from the old chain to the new one (including the elusive Satoshi Nakamoto, the Bitcoin developer who controls wallets containing around $100 billion of the cryptocurrency). If Q-Day happens before the hard fork, there’s nothing to stop bitcoin going to zero. “It’s like a time bomb,” says Dhiman.

The best-case scenario looks something like Y2K, where we have a collective panic, make the necessary upgrades to encryption, and by the time Q-Day rolls around it’s such an anticlimax that it becomes a joke. That outcome may still be possible. Last summer, NIST released its first set of post-quantum encryption standards. One of Joe Biden’s last acts as president was to sign an executive order changing the deadline for government agencies to implement NIST’s algorithms from 2035 to “as soon as practicable.”