 There is no the mempool. We use that terminology. We say it goes back into the mempool, but there isn't just one mempool. Every node has a mempool. Every node has its own queue of unconfirmed transactions that it's keeping track of. Miners use their nodes mempool to decide what to include in blocks. Wallets use their known nodes mempool to see if money has been spent but is yet unconfirmed or if there's a payment incoming but hasn't been confirmed. And to keep track of transactions that are pending, waiting in their mempool. If you were to take a snapshot of all of the different mempools out there, if that was possible, all of the decentralized mempools, almost all of them would have an overlap of better than 99.99%. So that out of maybe 10,000 transactions that are in the mempool, maybe 9,900 transactions are the same or 9,999 transactions are the same. And they only differ by a few. So even though the mempool is a decentralized structure which is held in each node, they more or less have the same stuff. And what happened in this rollback was that every node that then deprecated the 26 blocks that had previously been advertised that it was unable to confirm would then put all of those transactions back in their own mempool or would connect to the network and receive those transactions from other nodes that had kept them and repotulate its own mempool and see that a big queue had built up.