The 21e8 meme refers to a specific Bitcoin block hash (00000000000000000021e8...) found in block 528249, which many interpreted as a cryptic signal. The '21' corresponds to Bitcoin's 21 million supply limit, while 'e8' refers to the E8 Lie group, a complex mathematical structure used in Garrett Lisi's An Exceptionally Simple Theory of Everything. This coincidence sparked wild conspiracy theories across Twitter and Reddit, ranging from Satoshi Nakamoto being a time traveler to the universe being a simulation. Today, it remains a hallmark of deep-lore crypto culture, used to signify high-level intelligence or esoteric connections in the blockchain space.