Bitcoin Gold (BTG) hard fork

On October 24, 2017, Bitcoin forked at block 491,407 to create Bitcoin Gold, swapping SHA-256 for the GPU-friendly Equihash in a bid to wrest mining away from ASICs and back toward ordinary hardware. It briefly cracked the top tier of crypto market caps and handed every BTC holder a free airdrop, cementing the post-BCH template of fork-and-distribute that defined late 2017. (The public network and trading didn't actually go live until November 12.)