Bitcoin’s Mining Difficulty Is Doing Something Unusual This Year (
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"Bitcoin’s mining difficulty is adjusted roughly every two weeks to factor in the hashing power of new miners on the network. While the general trend has been upward almost since the network’s inception, it’s common for there to be brief but significant dips. In December of 2018, for example, mining difficulty dropped three straight times in a row, for a total drop of almost 30%."