Bitcoin had its third single-block reorganization in about four weeks in mid-September, and it put a question back in front of the industry: how concentrated have mining pools become? The reorgs themselves were minor. The concentration behind them is the part worth understanding, especially because one piece of it is something you, as a miner, decide directly.
What actually happened
Around mid-September, two pools found a valid block at nearly the same height. AntPool's block stayed in the main chain and SpiderPool's was discarded, which cost SpiderPool the roughly 3.14 BTC reward for that block. Galaxy Research noted it was the third such single-block reorg since mid-August.
This kind of reorg is normal, not an attack. It happens occasionally when two miners find blocks at almost the same moment, the network briefly sees two candidates, and it converges within seconds on the one with more accumulated work. The losing block becomes stale. That is a routine consequence of a global network with real propagation delay, and it is completely different from a malicious deep reorg, where an attacker with enough hashrate deliberately rewrites several blocks to reverse a payment. Bitcoin's main chain has never suffered one of those. What analysts flagged was not any single reorg but the pattern of three close together, which is a reasonable thing to notice without being a sign of an attack.
The real story is concentration
The reason near-simultaneous blocks and pool politics get attention is that block production is concentrated in a handful of pools. As of September 2026, the top two pools, Foundry USA and AntPool, produce roughly 45% of all blocks between them, and the top four produce around 70%, according to mempool.space and Hashrate Index. That is a lot of the network flowing through a short list of operators.
It may be more concentrated than those headline numbers suggest. The researcher known as 0xB10C has documented that several nominally independent pools broadcast block templates nearly identical to AntPool's, which points to shared infrastructure he calls "proxy pooling." Importantly, he is careful to say it is not proven that a single entity controls those pools. So treat it as a real, measured similarity in how some pools build blocks, and an open question about who is really behind them, rather than settled fact. If those pools do act as one bloc, effective concentration is higher than the published shares.
Why concentration matters
Concentration does not break bitcoin, but it creates two risks worth naming. The first is transaction censorship. If a small number of pools decide which transactions go into blocks, that is a small enough group to pressure through regulation or policy. The second is the coordinated deep-reorg or 51% scenario, where enough hashrate colluding could in theory rewrite recent history. Both are risks and scenarios, not things that have happened on Bitcoin's main chain, and it is worth being precise about that. The point is that a more spread-out set of pools makes both harder, and a concentrated one makes them less far-fetched.
What you actually control
Here is the part that matters for anyone running machines. Pool share is not ownership of the network. That hashrate belongs to thousands of independent miners who point it at a pool by choice and can repoint it at any time. Concentration lives at the coordination layer, not the ownership layer, which means it can shift as fast as miners decide to move.
You are one of those miners. Which pool you point your hashrate at is a real decision, and switching costs you nothing but a configuration change. Beyond the big pools, there are deliberately decentralized options: Ocean and solo mining let you avoid adding to the leaders, and Stratum V2's job negotiation lets a miner running its own node build its own block template, choosing its own transactions rather than letting the pool dictate them. Pools representing most of the network's hashrate have signaled support for open block-construction standards, so this is a live shift, not a theory. None of this requires you to be an activist. It just means the concentration numbers are partly a product of where miners casually point, and you can point somewhere else.
Where hosting fits
One thing worth clearing up, because people assume the opposite: hosting your miner does not hand your pool choice to anyone. In a real hosting arrangement, you own the hardware, you point it at the pool you choose, and the bitcoin pays out to your own wallet. The host provides power, space, cooling, security, and maintenance for a fee, and does not take custody of your coins. That is different from cloud mining, where you rent hashrate and own no machine. A hosted miner keeps full control of both pool and wallet, so hosting does not concentrate pool power. If anything, it frees you to make the pool decision on its merits instead of around your electricity bill.
Fathom Labs hosts at $0.07/kWh all-in in Southeast Texas, with no performance fees, on Whatsminer hardware you own and point wherever you like. If you want to talk through a setup, reach out.
Frequently asked questions
What is a bitcoin block reorg?
A reorganization, or reorg, happens when the network briefly has two competing versions of the most recent block and then settles on one, discarding the other. Short one-block reorgs occur occasionally when two miners find a valid block at nearly the same time, and the network converges within seconds on the block with more accumulated work. These are routine and benign, and are not the same as a malicious deep reorg or 51% attack, which has never happened on Bitcoin's main chain.
How concentrated are bitcoin mining pools?
As of September 2026, the top two pools, Foundry USA and AntPool, produce roughly 45% of all bitcoin blocks combined, and the top four produce around 70%, per mempool.space and Hashrate Index. Researcher 0xB10C has documented that some nominally separate pools broadcast nearly identical block templates, suggesting shared infrastructure, though he notes single-entity control is not proven. So real concentration may be somewhat higher than the published shares.
Why does mining pool centralization matter?
Concentration raises two risks. A small group of pools deciding which transactions enter blocks could enable censorship or be pressured by regulation, and enough colluding hashrate could in theory attempt a coordinated deep reorg. Both are scenarios rather than events that have occurred on Bitcoin's main chain. A more distributed set of pools makes both harder, which is why pool choice by individual miners matters for network health.
Does a hosted miner choose its own pool?
Yes. In a standard hosting arrangement, you own the hardware and configure it to your own chosen pool and wallet, and the host only provides power, space, and operations. The mined bitcoin pays out to your wallet, not the host's. This is different from cloud mining, where you rent hashrate and own nothing. Hosting keeps pool choice and payouts entirely in your hands. Fathom Labs hosts at $0.07/kWh all-in with no performance fees.
Thinking about where to point your hashrate, or where to host it? Talk to us and we will help you weigh the options.