How Mining Actually Works, Slowly
Mining is the most misunderstood word in Bitcoin — it suggests digging, and coins being "found." Nothing is dug up. Mining is a continuous, worldwide contest that does three jobs at once: it decides who writes the next page of the ledger, it makes cheating history expensive, and it issues new bitcoin on a schedule nobody controls.
This page takes the mechanism apart slowly. By the end, "proof of work" will be a thing you can picture.
The job: writing the next page
Recall the shape of the system: transactions wait in the mempool, and roughly every ten minutes a batch of them — a block — is sealed into the chain. Someone has to be chosen to do the sealing, and the choice has to work among strangers who may be dishonest. Elections can be rigged; lotteries need a trusted ticket-issuer. Bitcoin’s answer is a contest of work: prove you burned real computation, and your block may be written.
Miners collect waiting transactions, add their own special transaction paying themselves the block reward, and then begin the actual work — a guessing game played at staggering speed.
The guessing game, honestly described
Each miner repeatedly hashes its candidate block — runs it through the fingerprint function from the glossary — while changing one throwaway number called a nonce. The goal: a fingerprint that begins with enough zeroes. Hashes are unpredictable, so there is no strategy and no shortcut; a miner simply tries billions of fingerprints per second until one qualifies, or another miner finds one first.
It helps to picture a planetary dice tournament where the dice have more faces than atoms and you must roll below a target number. Skill is absent; speed and electricity are everything. When a miner wins, it broadcasts the block, every node verifies the winning fingerprint in an instant (checking is easy; finding was hard), and the contest restarts for the next page.
Difficulty: why it is always ten minutes
If more miners join, blocks would arrive faster — so every 2,016 blocks (about two weeks) the network automatically adjusts the target: more competition, more zeroes required; less, fewer. This difficulty adjustment is why the pace has held near ten minutes for over fifteen years while mining grew from laptops to industrial warehouses.
It is also the quiet answer to "what if everyone mines at once?" Nothing breaks. The game just gets proportionally harder, the way a lottery with more players does not produce more winners per draw — only more tickets.
The reward: where new bitcoin comes from
The winning miner’s block contains one special transaction that creates new bitcoin from nothing — the block reward, plus the fees from every transaction included. This is the only way bitcoin enters existence, and the amount halves roughly every four years: 50 BTC per block in 2009, and 3.125 since April 2024.
Because blocks arrive on a schedule and the reward shrinks on a schedule, the total supply approaches — and never exceeds — 21 million, with the final sliver due around 2140. After that, fees alone pay the miners. No committee sets this; it is arithmetic the whole network enforces.
Why the expense is the point
To rewrite a settled block — say, to erase a payment — an attacker would need to redo that block’s work and outpace the entire honest network thereafter, commanding more computing power than everyone else combined. The cost of attack scales with the cost honest miners already spend. Proof of work converts electricity into a wall around history.
You can reasonably ask whether the wall is worth its price — that is the energy debate, and reasonable people disagree. But it is a wall, not waste heat with a logo: remove the expense and you remove the thing that lets strangers trust a ledger none of them controls.
Mining, demystified, is a very expensive, very fair lottery whose prize is the right to be audited by everyone. That such a strange machine has run without pause since 2009 is either a warning or a wonder, depending on the observer. The machine itself is indifferent: it is already working on the next block.
Written by Robert King for Bitcoin in 15 Minutes. Educational only — not financial advice. Spotted an error? Tell me and I will fix it.