The Bitcoin network has just recorded a rare event: a 2-block reorg—when the blockchain temporarily splits before merging back together.
The event involved major mining pools such as Foundry USA, AntPool, and ViaBTC, highlighting direct competition among “hashrate giants.”
So what is a reorg, why does it happen, and is it dangerous?
Key Takeaways
- Bitcoin experienced a 2-block reorg — rare but normal
- Multiple miners found blocks at the same time → temporary fork
- Foundry won due to higher hashrate
- Blocks from AntPool and ViaBTC became stale blocks
- Transactions were not lost; they returned to the mempool
What is a Bitcoin reorg?
Definition of reorg
In the Bitcoin network, a reorg (chain reorganization) is when the blockchain temporarily splits into multiple branches (forks), then automatically selects one branch as the “main chain.”
Simply put:
- The blockchain is not always a single straight line
- Sometimes there are two parallel versions of history for a short time
- Eventually, the network keeps the stronger version (with more Proof-of-Work)
Result:
- One branch becomes the main chain
- The other is discarded (stale/orphan chain)
How does a reorg happen?
At the same moment:
- Miner A finds block #100
- Miner B also finds block #100
At this point:
- The network splits into two branches
- Some nodes see A’s block
- Others see B’s block
Then:
- Miner C builds block #101 on A’s block
- Miner D builds block #101 on B’s block
Now two chains compete:
- Chain A: 100 → 101
- Chain B: 100 → 101
Finally:
- If one side grows faster (e.g., Chain A reaches #102, #103…)
The network will:
- Choose Chain A as the main chain
- Discard Chain B
That process is a reorg
Why do reorgs happen?
Reorgs are not bugs—they are a natural result of Bitcoin’s decentralized design.
1. Independent miners
In Bitcoin:
- There is no central server
- Thousands of miners operate independently worldwide
This means:
- Multiple miners can discover valid blocks at nearly the same time
2. Network latency
Data doesn’t propagate instantly:
- When a block is mined
- It takes milliseconds to seconds to reach the whole network
During that time:
- Other miners may not yet know about the new block
- They continue mining their own version
This is the direct cause of temporary forks
3. “Longest chain wins” rule
Bitcoin follows the principle:
The chain with the most Proof-of-Work is accepted
This means:
- It’s not about who was first
- It’s about which chain has more total hash power
4. Probabilistic nature of mining
Mining is:
- Based on probability
- Impossible to predict exactly who will find the next block
So:
- Collisions between miners are unavoidable
When do reorgs happen more often?
Reorgs are more likely when:
- Many large miners are competing
- Hashrate distribution is uneven
- Network latency is higher than usual
Common types of reorgs
1-block reorg (most common)
- Happens fairly often
- Usually has no impact
2-block reorg (rare)
- Requires consecutive blocks on a competing chain
- Still considered normal
Long reorg (dangerous)
- May be linked to:
- 51% attacks
- Double-spending
Timeline of the 2-block reorg event on Bitcoin
This recent 2-block reorg on the Bitcoin network is a textbook example of how mining pools compete in real time. Here’s a step-by-step breakdown:
1. Fork begins at the same block height
At block height 941,881:
- Multiple major mining pools discovered valid blocks almost simultaneously
- All blocks were valid under Bitcoin rules (correct hash, valid transactions)
Pools involved:
- Foundry USA
- AntPool
- ViaBTC
Due to network latency:
- Not all nodes received the same block at the same time
- Some nodes saw Foundry’s block
- Others saw AntPool’s block
Result:
- The blockchain temporarily split into multiple branches (forks)
- Each branch represented a different “version of history”
2. Parallel chain development
After the fork:
Branch 1 – AntPool + ViaBTC
- AntPool mined the first block
- ViaBTC extended it by mining the next block
This created a kind of implicit cooperation:
- ViaBTC built on AntPool’s chain
- Increasing its chances against Foundry
Branch 2 – Foundry (independent)
- Foundry USA continued building its own chain
- Ignored AntPool’s branch
At this stage:
- Multiple chains existed in parallel
- Different nodes believed in different “correct” chains
This is a temporary consensus split
3. Foundry “wins” with higher hashrate
The situation changed when Foundry USA:
- Mined several consecutive blocks
- Rapidly increased total Proof-of-Work (PoW) on its chain
Why did Foundry have the advantage?
- It is the largest mining pool (~30%+ hashrate)
- Higher probability of mining the next block
In mining:
- It’s not about who is first
- It’s about who is faster over time
Deciding rule: “Longest Chain Wins”
In Bitcoin:
The winning chain is the one with:
- More blocks
- Greater total PoW
Once Foundry:
- Extended its chain beyond the others
The entire network:
- Switched to Foundry’s chain
- Abandoned the AntPool/ViaBTC chain
4. Other blocks become “stale blocks”
After Foundry’s chain was accepted:
Blocks mined by:
- AntPool
- ViaBTC
Were excluded from the main chain
What is a stale block?
A stale block is:
- A valid block
- But not included in the final accepted chain
Consequences of stale blocks
For miners:
- No block reward
- Loss of block subsidy + transaction fees
For the network:
- No errors
- No data loss
What happens to transactions in discarded blocks?
- They are not lost
- They return to the mempool
- They will be included in future blocks
This is a key safety mechanism of Bitcoin
What role does hashrate play in a reorg?
In the Bitcoin network, hashrate (computational power) is the key factor that determines who wins and who loses during a reorg.
Hashrate determines the probability of winning
Hashrate can be understood as:
- The number of hash calculations a miner performs per second
- A measure of mining power
In the Proof-of-Work mechanism:
- Miners don’t “choose” blocks
- They compete using computational power
Why is hashrate important in a reorg?
When a fork happens:
- Multiple chains exist simultaneously
- Each chain must continue being extended with new blocks
At that point:
- A pool with higher hashrate →
- Mines blocks faster
- Extends its chain faster
- Accumulates more Proof-of-Work
Core rule: “The strongest chain wins”
In Bitcoin:
It’s not the chain that appears first that wins It’s the chain with the greater total Proof-of-Work
Which means:
- Higher hashrate → higher probability of winning
Comparing mining pool strength
In this event:
- Foundry USA: ~32%
- AntPool: ~15%
- ViaBTC: ~7%
This implies:
- Foundry had a probability of mining blocks roughly:
- ~2× higher than AntPool
- ~4–5× higher than ViaBTC
In a short “block race”:
- Foundry had a huge advantage
- Just a few faster blocks were enough to dominate the competing chain
Important note: bigger pools don’t always win
Even with higher hashrate:
- Mining is still probabilistic
- Smaller pools can:
- Find blocks first
- Temporarily lead
But in the long run:
- Larger pools almost always have the advantage
Are transactions in discarded blocks lost?
Answer: NO
This is one of the most common misconceptions about reorgs.
What happens to those transactions?
When a block is removed from the chain:
- Transactions are not deleted
- They do not disappear
Instead:
- They return to the mempool (unconfirmed transaction pool)
What happens next?
- Miners will reselect these transactions
- Include them in new blocks on the main chain
Impact on users
Users will:
- Not lose funds
- Not lose transactions
Only experience:
- Delays in confirmation
- Possibly needing 1–2 more blocks
When should you be cautious?
If you:
- Send large transactions
- Or make important transfers
You should wait:
- 3–6 confirmations
To ensure safety against reorgs
Is a 2-block reorg dangerous?
Normal in Bitcoin’s design
Small reorgs (1–2 blocks):
- Are natural
- Caused by:
- Network latency
- Mining competition
They are:
- Not bugs
- Not hacks
Why small reorgs are not dangerous
Because:
- The network quickly resynchronizes
- Only one chain is finalized
- No fraud or data loss occurs
When does a reorg become dangerous?
1. Long reorgs (5–10+ blocks)
This may indicate:
- A powerful entity
- Capable of rewriting transaction history
2. 51% attack
If a group controls >50% of hashrate:
They can:
- Build a private chain
- Release it to replace the main chain
Serious consequences
Double spending
- Send Bitcoin → receive goods
- Then reorg the chain → erase the transaction
The seller:
- Loses money
- Cannot reverse it
Loss of market trust
- Users may fear the blockchain is no longer immutable
- Market prices could be affected
FAQ – Frequently Asked Questions
Is a reorg a bug in Bitcoin? → No. It is a normal mechanism of the blockchain.
Does a reorg cause loss of Bitcoin? → No. Transactions may be delayed, but they are not lost.
Should you worry about a 2-block reorg? → No. It’s a rare but normal occurrence.
Disclaimer:The information provided here is for informational purposes only and should not be considered financial, investment, legal, or professional advice. Always conduct your own research, consider your financial situation, and, if necessary, consult with a licensed professional before making any decisions.