Mining difficulty adjusts automatically every block. The graphs below show the difficulty time series for the last 1008 blocks.
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.