Bitcoin mining concentration exposed by rare 2-block reorg

Bitcoin mining concentration surfaced on-chain after Foundry USA, the largest BTC mining pool, produced seven consecutive blocks and triggered a rare two-block reorganization. Two valid blocks mined by AntPool and ViaBTC were orphaned, though their transactions returned to the mempool and were eligible for inclusion in later blocks. At block height 941,881, AntPool and Foundry found blocks within 12 seconds, creating a brief split in the chain. ViaBTC then extended AntPool’s side at 941,882, while Foundry kept extending its own chain. When blocks 941,883 through 941,886 all went to Foundry, the network selected the heaviest chain by cumulative proof of work, as designed. The event is not a Bitcoin security threat; a two-block reorg resolves within minutes under consensus rules. However, it is a clear on-chain signal of Bitcoin mining concentration as margins tighten, pushing smaller miners out and concentrating hashrate into fewer pools. Context: mining difficulty dropped 7.76% on Saturday, and total hashrate has reportedly fallen from a 2025 peak near 1 zettahash to about 920 EH/s. The key trading takeaway is that higher hashrate concentration can increase the probability of single pools mining multiple blocks in a row and creating short-lived competing chains when large pools find blocks nearly simultaneously.