Electricity
Humans mastered all elements before that, we learned how to create and destroy fire, how to change course of rivers, how to irrigate lands that are dry, we mastered Earth with agriculture and Forestry, we mastered the wind with sailing, windmills and so on.
Electricity was the only thing left for huge amount of years, it seemed as an impossible thing to master. Humans used to consider lightning as a deity, the strongest one is most mythologies.
And we finally learned how to master it and use it, IMO it started a new era in humanity, everything we learned before that was amplified with electricity.