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

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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 ultimately extended its competing branch further than rival mining pools AntPool and ViaBTC, causing their blocks to be dropped from the canonical chain, according to onchain data and observers including Bitcoin developer "b10c."

The event began when multiple miners found valid blocks at nearly the same time, creating a temporary fork in the network. Foundry mined its own version of block 941881 while AntPool mined a competing block at the same height, 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 therefore the one accepted by nodes that saw and validated it. As a result, the competing blocks mined by AntPool and ViaBTC became stale, meaning they were valid blocks that ultimately fell outside Bitcoin's canonical ledger.

Foundry's size may have influenced the odds of that outcome. 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. b10c's analysis of mining pool behavior during forks notes that 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 hashrate, compared with 15.7% for AntPool and 7.2% for ViaBTC, The Block's data dashboard shows.

How the two-block reorg unfolded

A two-block reorg occurs when a temporary tie between competing branches persists across more than one block before being resolved. While single-block reorganizations occur from time to time, a two-block event is less common but still a known outcome of near-simultaneous block discovery and ordinary network propagation, 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, restoring a single canonical history. The displaced blocks are often called stale blocks, though they are sometimes loosely referred to as orphaned blocks. The more precise term here is stale, because the blocks were valid and had known parents. However, the miners who produced them do not receive rewards.

Importantly, transactions included in those stale blocks were not necessarily lost. They either already existed in the winning chain or, if absent from it, returned to the mempool for possible inclusion in later blocks. The episode demonstrates Bitcoin's built-in mechanism for resolving temporary forks, with the network operating as designed by favoring the chain with the greatest cumulative proof of work.

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