What to know : Foundry USA, Bitcoin's largest mining pool, briefly produced seven consecutive blocks on Monday, triggering a rare two-block chain reorganization that orphaned valid blocks from AntPool and ViaBTC. The incident, which the network resolved as designed by favoring the chain with the most cumulative proof of work, underscores how shrinking industry margins are concentrating hashrate into fewer, larger mining pools. While a two-block reorg does not threaten Bitcoin's overall security, growing hashrate concentration increases the odds that a single pool will mine multiple blocks in a row and create short-lived competing chains when blocks are found nearly simultaneously.
Bitcoin's mining concentration problem just showed up on the blockchain itself, triggering a small "reorg."
Foundry USA, the largest bitcoin mining pool, produced seven consecutive blocks late on Monday and in the process orphaned two valid blocks mined by AntPool and ViaBTC.
Think of it as two checkout lines opening at the same time in a busy store. At first, both lines are moving, but suddenly, one of the line starts clearing customers faster. This leads everyone to shift to the faster line and the slower one gets abandoned.
That’s essentially what happened here: Dominant pool Foundry’s “line” moved ahead quickly with several blocks in a row, so the network followed it, leaving the other valid blocks by AntPool and ViaBTC behind.
Bitcoin miners compete to add new blocks of transactions to the blockchain, and sometimes two miners find a valid block at nearly the same time. When that happens, the network briefly splits, but it ultimately chooses one chain to continue – usually the one that grows faster.
A mining pool, such as Foundry, is a group of miners who combine their computing power to mine blocks and split the rewards,. Finding a block solo is like winning a lottery that individual miners can rarely win on their own.
Bitcoin's consensus rule is absolute: the chain with the most cumulative proof of work wins. AntPool and ViaBTC's two blocks became stale, permanently erased from the ledger, and those miners earned nothing for producing them.
The event was a 2-block chain reorganization, rare but not unprecedented, and the clearest on-chain signal yet that hashrate is concentrating into fewer hands as the industry contracts.
At block height 941,881, AntPool and Foundry found valid blocks within 12 seconds of each other, at 15:49:35 and 15:49:47 UTC respectively. Both were legitimate and the network briefly split, with some nodes following one chain and others following the other.
The race continued to block 941,882, where ViaBTC extended AntPool's chain and Foundry extended its own.
That created two competing chains, each two blocks deep, running in parallel. Later on, blocks 941,883 through 941,886 all went to Foundry, making their chain the heaviest by a wide margin.
Transactions in the orphaned blocks weren't lost, however. They return to the mempool and get included in future blocks. An orphaned block is a valid block that loses the race when two miners find blocks at nearly the same time, getting discarded permanently from the chain despite being perfectly legitimate.
Mining difficulty just dropped 7.76% on Saturday, the second-largest negative adjustment of 2026. Hashrate has retreated to roughly 920 EH/s from the 1 zetahash record hit in 2025.
Smaller and mid-sized miners are exiting because bitcoin at $70,000 sits well below the estimated $88,000 average production cost. Every operator that shuts down concentrates the remaining hashrate into fewer pools.
A 2-block reorg doesn't threaten Bitcoin's security. The network handled it exactly as designed, with the longer chain winning and consensus re-establishing within minutes.
But when fewer pools control more hashrate, the probability of a single pool finding multiple consecutive blocks increases, and with it the probability of competing chains when two large pools find blocks near-simultaneously.
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