Comment: 17 pages, 11 figures. For associated model, see: https://github.com/davidelasi/bitcoinThe mining of bitcoin is modeled using system dynamics, showing that the past evolution of the network hash rate can be explained to a large extent by an efficient market hypothesis applied to the mining of blocks. The possibility of a decrease in the network hash rate from the next halving event (May 2020) is exposed, implying that the network may be close to 'peak hash', if the price of bitcoin and the revenues from transaction fees will remain at approximately the present level.