So, let’s suppose that this limited level of proof was worth it to you. You are willing to pay such that future generations can see an indelible record of a bit of history. How would you get that proof onto the Bitcoin blockchain? The way you’d do this is by organizing your arbitrarily large external dataset (a photo, or something much larger than that) into a Merkle tree, calculating a string of fixed length called a Merkle root, and then writing that to the Bitcoin blockchain through OP_RETURN. This furnishes a tool for proof-of-existence for any digital file. You can do this as a one-off for a single piece of data, or as a periodic backup for any non-Bitcoin chain. So you could, in theory, put a digital summary of many gigabytes of data from another chain on the Bitcoin blockchain every ten minutes for the price of a single BTC transaction, thereby proving it existed