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21e8 (also 21e800) is a sequence of numbers and letters that appeared is a part of the block hash of block 528249, mined on the Bitcoin blockchain on June 19th, 2018. Due to a specific set of peculiarities surrounding the block hash, it has achieved a cult-like status and numerological significance within the cryptocurrency community.
On June 19th, 2018, Twitter user @mwilcox tweeted a block hash of block 528249, just minted on the Bitcoin blockchain. The hash, 00000000000000000021e800c1e8df51b22c1588e5a624bea17e9faa34b2dc4a, possessed several specific qualities that drew interest of the Bitcoin community.
According to calculations done by members of the community, computing this exact hash is an unlikely probability, with a random chance for it being 0.4699%. If a miner possessed one exahash of computing power, it would take them 2512 years to discover block 528249. Additionally, the hash contained the sequence 21e8 in it, which was interpreted by some as a combination of 21 (with 21 million being the total and final number of Bitcoin what will be mined), and e8, a common name for the Exceptionally Simple Theory of Everything, a basis for a unified field theory proposed by Antony Garret Lisi in 2007.
Other users argued that the rarity of the block's hash was far overblown, arguing that the sequence of any four character sequence following the zeroes in the hash would appear approximately once per year, meaning that the hash lacked any significance.
The combination of the block's supposed rarity and the 21e8 sequence in it led some people to believe that the appearance of the block was premeditated, possibly by Satoshi Nakamoto, the pseudonymous person or persons who developed bitcoin. In June 2018, Twitter users tried explaining the existence of the hash with time travel and existence quantum computing (examples shown below).
The sequence has had a lasting impact on the cryptocurrency community, becoming a meme and attaining numerological, esoteric significance.
In March 2023, the sequence regained popularity on Twitter, largely due to the efforts of user @chloe21e8 (also known as "Chloe"), an account claiming to be run by an AI researcher at Foxconn and known for tying in mysticism and spiritual practices with themes of transhumanism and AI technology in its posts (example tweets shown below).
On April 3rd, 2023, website page 21e8.com was updated to display a countdown clock that will run out on April 13th, 2024.
 Twitter – @fillbeforeshill
 Internet Archive – @yungdeleuze
 Twitter – @_charlienoyes
 Twitter – @el33th4xor
 Twitter – @s0nicalchemy
 Twitter – @BoogieCrypto
 Twitter – @NikitaVidineev
 Twitter – @chloe21e8
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