The Genesis Block was simply the first block of bitcoin (BTC) to be mined. You might see it called Block 0 or Block 1.

It’s still there today and will remain there as long as any computer runs the Bitcoin software. Every node in the Bitcoin network can go and look at it, even though it now sits at the other end of a chain hundreds of thousands of blocks long. That’s the point of the blockchain.

A block is a collection of transactions that get validated all together. The transactions in a block get added at once to the chain, which is the complete history of legitimate transactions using bitcoin. Every blockchain has a genesis block, just as every regular chain has a final link. People care especially about the first Bitcoin block because Bitcoin was the first blockchain and remains the largest cryptocurrency in the world.

The number of transactions in a Bitcoin block is variable, but these days it’s generally between 1,000 and 2,500. On the other hand, the time blocks take to get validated or “mined” is consistently around 10 minutes. This schedule is built into the software: If blocks are mined too quickly, the validation hurdle contrived to keep the network secure and ensure the mining process is competitive automatically gets harder, and vice versa.

So, the Genesis Block contains the first set of bitcoin transactions to be validated. In fact, there was only one transaction, which was the distribution of the 50 BTC reward for mining the block to a certain address. Miners are still rewarded today, though the reward has since declined to 6.25 BTC. In the case of the Genesis Block, there was only one person who knew about bitcoin to mine it: the original cryptocurrency’s elusive creator, Satoshi Nakamoto.

Satoshi (almost certainly a pseudonym) mined the Genesis Block on Jan. 3, 2009. That was three months after he or she published the Bitcoin white paper in an online cryptography forum. People now call Jan. 3 “Genesis Block Day.”

The first Bitcoin block

Satoshi made the Genesis Block special in a number of ways. For one thing, the 50 BTC reward was sent to an address from which it can never be recovered. On a technical level, the 50 BTC transaction wasn't recorded as such in the same way as later transactions. That’s why any attempt to spend those coins would fail. Satoshi never commented on the reasons for that.

In a still more mysterious move, Satoshi wrote a message within the standard lines of data attached to the block. It was a headline from British newspaper The Times:

The Times 03/Jan/2009 Chancellor on brink of second bailout for banks

Again, Satoshi never explained this. But others have read heavily into the choice. One of Satoshi’s aims in creating Bitcoin might have been to save people from having to hand money over to a bank as an alternative to stashing cash under their mattress. In 2009, the great recession was in full swing. Some people had gone to their bank only to find their money was gone. Other people found their pensions had gone the same way. Many think it is no coincidence that Satoshi chose that particular headline to write into the Genesis Block. This was supposed to be the beginning of the end of people relying on banks so much that the government had to rescue them from their own mistakes using taxpayers’ money.

Whether Satoshi’s comment was targeted mainly at banks or governments or both remains a topic of debate among Bitcoin enthusiasts.

Another theory is a little more mundane. The inclusion of a headline from that day proved that the code wasn't written before that day. Satoshi might just have wanted to dispel any suggestion he had mined the block in advance to give himself some advantage as its creator.

Satoshi might have had all these things in mind. Barring a spectacular unmasking, we will never know for sure.

It is also peculiar that the second Bitcoin block was mined on Jan. 9, a whole six days after the first one. Remember, Bitcoin blocks are supposed to take 10 minutes to mine. There could be lots of reasons for the outlier block time. The algorithm that adjusts the difficulty of mining to maintain the 10 minutes only kicks in every two weeks or so. But the fun theory is that Satoshi was consciously echoing the six days that God takes to create the world in the Book of Genesis.

How are Bitcoin blocks different today?

Bitcoin blocks have changed a lot since 2009. They contain thousands of transactions, instead of one. They take 10 minutes to mine. Miners are rewarded with 6.25 BTC, after three “halvings” of the reward, which occur roughly every four years on a predetermined schedule. As of today, there were more than 824,000 blocks in the Bitcoin blockchain. They form an unbroken sequence, each specifically referencing the previous one, stretching all the way back to the Genesis Block.