Bitcoin Mining Concentration Triggers Rare 2-Block Chain Reorganization

A rare but telling event recently played out on the Bitcoin blockchain, offering a real-time glimpse into the growing concentration of mining power. Foundry USA, currently the largest Bitcoin mining pool, produced seven consecutive blocks in a row, in the process leaving two valid blocks mined by AntPool and ViaBTC permanently orphaned and unrewarded.

To understand what happened, picture two checkout lines opening simultaneously at a grocery store. Both move at first, but when one suddenly clears customers much faster, everyone migrates to it and the other line empties out. Foundry's rapid streak of blocks worked the same way — the network followed the longer, heavier chain and abandoned the competing one.

At block height 941,881, AntPool and Foundry each found a valid block within just 12 seconds of each other, briefly splitting the network. ViaBTC then extended AntPool's chain while Foundry built on its own, creating two competing two-block chains running simultaneously. When Foundry claimed blocks 941,883 through 941,886 in succession, its chain became the dominant one by a decisive margin, and the other was discarded entirely.

Crucially, no transactions were lost. Orphaned block transactions return to the mempool and eventually get confirmed in future blocks. The network resolved exactly as Bitcoin's consensus rules intended — the chain carrying the most cumulative proof of work simply won.

Still, the event highlights a structural concern. Bitcoin's mining difficulty recently dropped 7.76%, the second-largest negative adjustment of 2026, as hashrate has pulled back to around 920 EH/s from a record 1 zetahash in 2025. With Bitcoin prices hovering near $70,000 — well below the estimated $88,000 average production cost — smaller miners are shutting down, leaving an increasingly consolidated landscape dominated by fewer, larger pools.

A 2-block reorganization poses no security threat to Bitcoin. However, as hashrate concentrates into fewer hands, situations like this become statistically more likely, making it a trend worth watching closely.