Bitcoin sees rare two-block reorg as Foundry overtakes AntPool and ViaBTC in mining race

Bitcoin experienced a rare two-block reorganization involving competing blocks at heights 941881 and 941882 on Monday, after rival mining pools briefly produced parallel versions of the blockchain.

Foundry USA Pool's branch ultimately won out over rival mining pools AntPool and ViaBTC, causing their blocks to be dropped, according to onchain data and observers including Bitcoin developer "b10c."

Miners in the respective pools found valid blocks near simultaneously, creating a temporary fork in the network. Foundry USA Pool mined its own version of block 941881 while AntPool mined a competing block, and ViaBTC extended that competing chain with block 941882. At the same time, Foundry continued building on its own branch, producing an alternative version of block 941882.

Bitcoin's consensus rules resolve these situations by selecting the chain with the greatest cumulative proof of work. Foundry then mined additional blocks, eventually producing seven in a row, making its branch the heaviest and accepted. As a result, the competing blocks mined by AntPool and ViaBTC became stale, meaning they were valid blocks that ultimately fell outside of Bitcoin's permanent ledger and the miners who produced them did not receive rewards.

Larger mining pools have a higher probability of finding consecutive blocks during short-lived forks, though that does not by itself indicate anything abnormal occurred as a pool's chance of finding the next block is equal to its share of network hashrate.

Foundry USA Pool accounts for roughly 32.2% of Bitcoin's total hashrate, compared with 15.7% for AntPool and 7.2% for ViaBTC, The Block's data dashboard shows.

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How the two-block reorg occurred

A two-block reorg occurs when alternative branches compete across more than one block before being resolved. While single-block reorganizations occur more regularly, a two-block event is less common but still a known outcome for the network, as noted by b10c in a post on the Bitcoin Network Operations Collective forum.

Once the Foundry branch pulled ahead, nodes that had been following the competing branch reorganized to the work-heavier chain. Importantly, transactions included in those stale blocks were not necessarily lost. They either already existed in the winning chain or were returned to the mempool for inclusion in later blocks.