http://btcexpz7xlwrqknqw6p3bok6shf73qtkd6zxct4x4qta2ktvh7ntprad.onion/difficulty-history
A decrease in difficulty indicates miners have left the network so finding each block is adjusted to be easier for the smaller number remaining. The numeric Difficulty is a multiple of the difficulty of finding the easiest block (Block #0) - e.g. blocks in BTC mainnet epoch 308 are over 16 trillion times harder to mine than those in epoch 0.