People have harnessed energy from moving water for thousands of years. Greeks used various types of water wheels to grind grain in mills more than 2,000 years ago. In the late 1800s, people figured out how to harness the power to produce electricity. Throughout the 20th century and into the 21st, hydropower has expanded, producing about 17 percent of the world’s electricity by 2014 and about 85 percent of renewable energy—and it shows no signs of slowing.