In this paper we revisit the mining strategies in proof of work based cryptocurrencies and propose two strategies, we call smart and smarter mining, that in many cases strictly dominate honest mining. In contrast to other known attacks, like selfish mining, which induce zero-sum games among the miners, the strategies proposed in this paper increase miners' profit by reducing their variable costs (i.e., electricity). Moreover, the proposed strategies are viable for much smaller miners than previously known attacks, and surprisingly, an attack performed by one miner is profitable for all other miners as well. While saving electricity power is very encouraging for the environment, it is less so for the coin's security. The smart/smarter strategies expose the coin to under 50\% attacks and this vulnerability might only grow when new miners join the coin as a response to the increase in profit margins induced by these strategies.