The cerebellum – sometimes known as the "little brain" – is located underneath the two hemispheres. It looks different from the rest of the brain because it consists of much smaller and more compact folds of tissue. It represents about 10 per cent of the brain's total volume but contains 50 per cent of its neurons.
This is an insane and fascinating finding. The cerebellum is a mystery to pretty much everyone, I think. I've been reading about and professionally studying the brain for close to a decade, and I can say with full confidence that I have very little idea what it does. People always say it has roles in this or that, without really articulating what it does--probably because they don't know either.