Bitcoin's network experienced three separate blockchain reorganizations in just four weeks, a pace researchers describe as unusual and symptomatic of a deeper centralization problem in mining. On September 11, 2026, SpiderPool and AntPool each mined valid blocks at height 966,500 within seconds of each other, triggering a brief network split that cost SpiderPool approximately 3.1389 BTC. This was the third such event in a month, following similar incidents on August 16 and August 24, and it has put a spotlight on a troubling statistic: the two largest mining pools now produce nearly 45% of every Bitcoin block mined.

What Exactly Happened During the September 11 Reorganization?

On September 11, 2026, at block height 966,500, SpiderPool and AntPool discovered valid blocks pointing to the same parent block within moments of each other, before either miner could confirm the other's existence. Both blocks propagated across different parts of the Bitcoin network simultaneously. According to Galaxy Research, which first flagged the event, its own node never saw the winning AntPool block until it had already been confirmed and extended on top of the chain. Bitcoin's consensus rule is straightforward: the chain with the most cumulative proof-of-work wins. Once a miner extended the AntPool block with a follow-up block, that side of the fork carried more total work, and every node on the network reorganized to it. SpiderPool's block became an orphan, and the 3.1389 BTC subsidy SpiderPool would have earned, worth approximately $240,000 at current prices, simply evaporated.

One-block reorganizations are not inherently new or dangerous. Bitcoin's difficulty adjustment and ten-minute block target exist precisely so the network can absorb occasional simultaneous block discoveries. What has changed is the frequency. Three reorgs in a four-week span is a higher rate than Bitcoin has seen in years of relatively quiet block production, and researchers who model Bitcoin's threat surface point out that the same concentration that makes these races more frequent also lowers the bar for something more deliberate.

How Does Mining Pool Concentration Threaten Bitcoin's Security Model?

Bitcoin's security model has always rested on a fundamental assumption: no single actor controls enough hashrate to rewrite history or censor transactions at will. As of mid-September 2026, that assumption is looking thinner than it has in years. Data from Hashrate Index shows the following breakdown of network hashrate distribution:

• Foundry USA: Approximately 209 exahashes per second (EH/s), representing 25.5% of network hashrate
• AntPool: Approximately 157 EH/s, representing 19.2% of network hashrate
• F2Pool: Approximately 119 EH/s, representing 14.5% of network hashrate
• ViaBTC: Approximately 87 EH/s, representing 10.6% of network hashrate
• SpiderPool: Approximately 66 EH/s, representing 8.1% of network hashrate

When you add up just the top two pools, Foundry USA and AntPool, they account for 44.6% of everything the network mines. The top four pools control nearly 70% between them. Research published by Spark puts Bitcoin's Nakamoto coefficient, the minimum number of entities that would need to collude to control the majority of hashrate, at just three pools. This figure has been sliding for close to a decade. In May 2017, the two largest pools together held less than 30% of hashrate; by December 2023, that combined share had already crossed 55%.

Is the Concentration Problem Even Worse Than It Appears?

The headline pool-share numbers likely understate the real concentration problem. Bitcoin Core contributor 0xb10c has documented that several pools that appear independent in block explorers are actually relaying jobs built by AntPool's parent company, Bitmain. According to detailed technical research, Poolin, CloverPool, Ultimus Pool, Binance Pool, SecPool, SigmaPool, Rawpool, Luxor, and Mining Squared all relay AntPool-constructed block templates rather than building their own. When that proxy network is counted as a single economic actor rather than nine separate pools, the combined share climbs well past what the public leaderboards show. Spark's research estimates that the true concentration is significantly higher than the raw pool statistics suggest, meaning the effective number of independent mining entities controlling Bitcoin's hashrate is even smaller than the headline figures indicate.

Why Are Mining Pools Consolidating Around AI and Data Centers?

The concentration of mining power is not happening in a vacuum. Bitcoin mining economics have deteriorated significantly, pushing major miners to diversify away from pure cryptocurrency mining toward artificial intelligence and high-performance computing (HPC) infrastructure. Listed Bitcoin miners and AI data center operators spent $30.7 billion in capital expenditures in the first half of 2026, surpassing the $21.53 billion recorded for all of 2025, a 42.6% increase. However, this massive spending has not translated into proportional mining revenue growth. Although HPC and AI revenue at nine major miners rose 52% quarter-over-quarter to $205.8 million in Q2 2026, capital expenditures during the same period were nearly 15 times the revenue generated by these businesses.

As listed Bitcoin miners accelerate the reallocation of power and infrastructure toward AI and HPC, the realized hashrate of the miners tracked fell 13.4% over six months, from 368.3 EH/s in Q4 2025 to 319 EH/s in Q2 2026. Over the same period, Bitcoin's network-wide quarterly average hashrate declined 10.6%. Excluding Bitdeer, which continued to expand against the trend, the realized hashrate of the remaining miners in the sample fell 21.2%. TheEnergyMag attributed the contraction mainly to weakening mining economics and growing competition between AI and Bitcoin mining for capital and power resources.

What Are the Practical Implications of Mining Centralization?

The concentration of hashrate among a small number of pools creates several risks that extend beyond occasional one-block reorganizations. A pool holding a large enough share of hashrate does not need a network-breaking bug to reorganize the chain; it just needs enough of an edge in block production to consistently win short-distance propagation races, or in a more adversarial scenario, to intentionally withhold and release blocks to reorg out a rival's work. While none of the three recent reorgs have been characterized as attacks, and each appears to be the byproduct of two large pools discovering blocks within moments of one another, the infrastructure now exists for more deliberate actions if an actor chose to pursue them.

The frequency of reorgs also has direct economic consequences. Each reorganization represents lost revenue for the miner whose block becomes an orphan. In the September 11 event, SpiderPool lost approximately $240,000 in block rewards. When these events occur three times in a month, the cumulative economic impact becomes material, and the incentive structure that keeps miners honest begins to shift.

Bitcoin's mining difficulty fell 0.74% in its latest adjustment to 126.23 trillion, down about 14% from its January 2026 peak and about 19.1% from its all-time high. The pressure on hashrate growth has been driven mainly by weaker BTC prices, declining mining revenue, and miners redirecting capital and power resources toward AI and high-performance computing. As mining becomes less profitable, the economic incentives that keep the network secure may weaken further, potentially accelerating consolidation among the largest, most efficient operators.

How to Understand Bitcoin's Mining Centralization Risk

• Pool Concentration Metric: Track the Nakamoto coefficient, which measures the minimum number of entities needed to control 51% of hashrate. As of September 2026, this number stands at just three pools, down from a healthier distribution in prior years.
• Hidden Concentration Problem: Monitor whether independent-appearing pools are actually relaying block templates from larger operators like Bitmain's AntPool, which effectively consolidates control beyond what public leaderboards show.
• Reorganization Frequency: Watch for clusters of one-block reorgs, which indicate that propagation delays between large pools are creating opportunities for simultaneous block discovery and network splits.
• Mining Economics Pressure: Follow whether declining mining profitability and competition from AI infrastructure continue to push smaller miners out of the market, further concentrating hashrate among the largest operators.

The September 11 reorganization and its two predecessors in August are not isolated anomalies. They reflect a structural shift in Bitcoin's mining landscape toward fewer, larger pools controlling a greater share of the network's computational power. While Bitcoin's consensus mechanism is designed to handle occasional simultaneous block discoveries, the increasing frequency of these events signals that the network's security assumptions may be under strain. As mining becomes less profitable and capital flows toward AI infrastructure, the incentive structure that has kept Bitcoin's network decentralized for over a decade is shifting in ways that researchers and developers are only beginning to fully understand.