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

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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.”