I think back in the day, every Bitcoin full node (using the Bitcoin Qt software) sort of functioned like a blockchain explorer. Good programmers could dig up more info about transactions directly from a local copy of the blockchain data. They did this using command-line tools, and often had to run specific commands to make the data readable.
This is essentially how theymos came up with the idea of making data easily viewable to the public, leading to the creation of the first blockchain explorer.
I created a site that lets you view detailed information about Bitcoin blocks, addresses, and transactions.
http://theymos.ath.cx:64150/bbe/This data is all gotten from the block chain. It has always been possible to get this data, but you had to patch Bitcoin with getblock or use the clunky -printblock switch in Bitcoin, and then you had to search through miles of data to get what you wanted. Now everything is available in a clickable and easily-searchable format.
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