Hydropower is the world’s largest source of renewable energy, yet we don’t hear much about it. That’s in part because hydropower generation in the U.S. has remained relatively steady for decades.

But internationally, it’s been growing rapidly, especially in China. That growth has some environmentalists concerned, as dams and reservoirs disrupt the surrounding ecosystems and can create C02 and methane emissions.

Plus, hydropower generation is threatened as climate-driven droughts become increasingly common.

Yet many experts say that hydropower is absolutely vital for a fossil fuel-free world and so learning how to mitigate these challenges will be critical in the decades to come.