What Is the Blockchain? Bitcoin's Public Ledger Explained Simply

The blockchain is a shared notebook that thousands of strangers keep in sync without trusting each other. Here is how the pages are made and why nobody can quietly rewrite them.

By Michal Beno, CEO, OneMiners21 October 2026

Picture a notebook of payments. Each page lists a few thousand transactions. When a page is full, it is sealed and a new one starts. Now imagine that thousands of people around the world each hold an identical copy, and every new page is checked by all of them before they add it. That notebook is the blockchain. The pages are blocks, and the people holding copies are nodes: computers running Bitcoin software.

The clever part is how the pages are sealed. Every page carries a fingerprint of the page before it. You cannot remove or edit a page in the middle without the fingerprints on all later pages failing to match. That is why it is called a chain of blocks, and why nobody, not a bank, a government or a big mining company, can quietly edit the record.

Bitcoin was described in a white paper published by Satoshi Nakamoto on 31 October 2008. The first block, called the genesis block, was created on 3 January 2009. Every block since points back, link by link, to that first one.

A block has two parts. The body is the list of transactions, usually a few thousand. The header is a short summary of exactly 80 bytes, and it is the part that miners hash over and over. The header has six fields.

Two of those fields do the heavy lifting for security. The previous block hash links the chain together. The Merkle root locks in the transactions. The nonce and target belong to mining, which is covered step by step in the OneMiners guide to how Bitcoin mining works.

A hash function takes any input and produces a fixed-length fingerprint. Bitcoin uses SHA-256, which always outputs 64 characters. The same input always gives the same output, but change even a single character and the output changes completely. There is no way to work backwards from a hash to its input.

Each block's identity is the hash of its header. Because the header includes the previous block's hash, every block's fingerprint depends on its parent, which depends on its parent, and so on back to 2009. Here are three real blocks mined on 5 October 2026:

Look at the pattern: block 970,001 stores the hash of block 970,000, and block 970,002 stores the hash of 970,001. If anyone changed one transaction in block 970,000, its hash would change, so block 970,001 would point to a block that no longer exists. Every node would spot the mismatch immediately.

A block can hold thousands of transactions, but the header has room for only one 32-byte summary of them. The Merkle root is that summary. It is built like a tournament bracket: hash every transaction, then hash them together in pairs, then hash the pairs, until one hash is left at the top. The idea is described in section 7 of the Bitcoin white paper and is named after computer scientist Ralph Merkle.

Here is a toy version with four made-up payments, which you can repeat with any SHA-256 tool. Then one amount is changed by a tiny 0.001 BTC.

Two things stand out. Changing one payment changed its leaf, its pair and the root, while the untouched branch stayed the same. And the root changed completely, so the block header changes, so the block hash changes, so the chain link to the next block breaks. The Merkle tree has a practical bonus too: a light wallet can prove a payment is in a block using only a handful of hashes along one branch, without downloading thousands of transactions.

Hashes make tampering visible. Proof of work makes it expensive. Each block hash must fall below a target, and finding one takes the whole network about ten minutes of guessing at about 970 EH/s, roughly 970 quintillion guesses per second. To rewrite an old block, an attacker must redo that work for the block and for every block after it, then overtake the honest chain, which keeps growing.

The Bitcoin white paper worked out the odds. If an attacker controls 10% of the network's hashrate, the chance of catching up falls sharply with every block added on top of a transaction. Each extra block is called a confirmation.

At six confirmations, about one hour, the white paper's chance falls to roughly 0.02%. That is why exchanges and merchants often wait a few confirmations for large payments. Rewriting deep history is not technically forbidden; it is just so costly that it makes no economic sense, because the same hashrate earns more by mining honestly.

Every Bitcoin transaction is public. Addresses are not names, so the ledger is pseudonymous rather than anonymous: you can see that an address moved coins, but not automatically who owns it. Anyone can run a node and download the whole chain, about 773 GB in October 2026, or use a free block explorer website that shows the same data in a browser.

A block explorer is the easiest way to see all this for yourself. Type in a block number, a transaction ID or an address and it shows the details. Popular explorers include mempool.space and blockchain.com.

The genesis block is a fun first search. It holds a single transaction, and its coinbase carries a newspaper headline from 3 January 2009 about a bank bailout, a timestamp proving the block could not have been made earlier.

Nodes keep and check the ledger. Miners write it. Every block in the chain was added by a miner who gathered waiting transactions, built a header with the right previous hash and Merkle root, and then found a nonce that made the header hash fall below the target. In return, the miner's first transaction pays it the 3.125 BTC subsidy plus the fees inside the block. Fees are explained in Bitcoin transaction fees explained.

That same work is what makes the ledger hard to rewrite. The more hashrate secures the network, the more an attacker would need to spend. In other words, the electricity miners use is not separate from the blockchain's security; it is the security.

For anyone who owns a Bitcoin miner, this is the job it does around the clock: building candidate blocks and hashing headers trillions of times per second. Its earnings depend on its share of the network's hashrate, and its costs depend mostly on power price. At today's numbers, hashprice is about $0.0398 per TH/s per day before fees.

Antminer S21 Pro 245T$2,299Hashrate245 TH/sEfficiency15.00 J/THPower draw3,675 WBreak-even power$0.111/kWhNet/day at 4.80c$5.52View on OneMiners

Antminer S21 XP Hyd 473T$6,199Hashrate473 TH/sEfficiency12.00 J/THPower draw5,676 WBreak-even power$0.138/kWhNet/day at 4.80c$12.29View on OneMiners

Antminer S23 Hyd 580T$12,299Hashrate580 TH/sEfficiency9.50 J/THPower draw5,510 WBreak-even power$0.175/kWhNet/day at 4.80c$16.74View on OneMiners

Three efficiency tiers on the OneMiners catalogue, live prices 6 October 2026. Card figures at $0.048 per kWh use hashprice including recent fees. At today's numbers, not a promise of future revenue.

Frequently asked questions

Michal Beno

CEO of OneMiners, the global Bitcoin mining hosting and hardware platform with 15 published hosting locations, 7-year fixed electricity contracts and a 7-year hardware warranty. He writes about the economics of industrial Bitcoin mining.

Informational only, not financial advice. Figures are a snapshot from public market data and news reports as of 5 October 2026 and can change quickly. Crypto prices can fall as well as rise. No return is offered or implied. Do your own research.