Bitcoin’s mining concentration problem just showed up on the blockchain itself, triggering a small “reorg.”

At the center of the story is Foundry USA, the largest bitcoin mining pool, representing a group of miners who combine their computing power to verify transactions, mine blocks, and split the rewards in BTC.

On the blockchain, there are many miners, and sometimes two or more find a block at nearly the same time. When that happens, the network temporarily has two competing versions of the blockchain. Eventually, the network reorganizes back into a single chain, depending on which version grows faster. This process is called a blockchain reorganization, or “reorg.”

That’s what happened on Monday: Foundry and AntPool both mined blocks at roughly the same time, causing a chain split. Foundry then produced several consecutive blocks, moving slightly faster than its competitors, and became the chain the network followed.

The result: the blockchain reorganized to Foundry’s version, and the blocks mined by AntPool and ViaBTC were orphaned or effectively erased from the ledger. Those miners earned nothing for the work they had done.

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.

The episode highlights the risks of mining concentration in Bitcoin and how controlling network power can translate directly into outsized influence and losses for rivals. When a single pool like Foundry can produce several blocks in a row, it can trigger a reorg, and orphan valid blocks from other miners.

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.

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.

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.