Mining difficulty adjusts automatically every block.
The graphs belove show the difficulty time series for the last 1008 with a granularity of 4, even thou actual difficulty change for every new block. This as been done to reduce the load on the explorer.
The adjustment aims to maintain an average block-mining time of 10 minutes.
A growth in the difficulty indicates that the average block-mining time during the previous 144 blocks was less than 10 minutes (due to more miners joining the network and 'searching' / 'mining' for blocks).
A drop in difficulty indicates miners have left the network so finding each block is adjusted to be 'easier' for the smaller number remaining.
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