Leonardo da Vinci was perplexed by the power of the ocean. Notebooks reveal he speculated that tides were caused by the breath of a huge underwater creature — and went on to attempt to calculate the size of its lungs. Generating three terawatts of energy a day, tidal power truly is colossal. Yet, compared to other renewable methods, it has yet to make an impact. Here, I'd like to explain the complications and potential of tidal energy.