NOVA's expert team looks for the signature of a volcanic eruption big enough to have blasted a huge cloud of ash and sulfuric acid into the atmosphere. Killer Volcanoes spotlights the search for the mystery volcano that plunged the globe into a deep freeze and inflicted famine on medieval Europe.

NOVA takes viewers inside the operating room to witness organ transplant teams transferring organs from donors to recipients. Meet families navigating both sides of a transplant, and researchers working to end the organ shortage. Their efforts to understand organ rejection, discover ways to keep organs alive outside the body, and even grow artificial organs with stem cells, could save countless lives.

In a spectacular adventure, NOVA unlocks the mystery on the vast, grassy plains of Kazakhstan, where horses still roam free, and nomadic herders follow their traditional way of life. Investigating clues from archaeology and genetics, researchers reveal vivid evidence of the very first horsemen. They also discover warriors who swept across Europe and turn out to be the ancestors of millions today.

Whether it's electric lights, addiction to digital devices, or just the fast pace of modern life, one thing is clear: it seems to be getting harder to get a good night's sleep. Almost 50 percent of Americans say they can't get the zzz's they need. To understand why many are sleep deprived, take a deep dive into the body's control center, the brain, and follow the work of researchers who are unraveling the secrets of sleep.

As global temperatures continue to rise, scientists are wondering if there are solutions beyond reducing emissions. Enter geoengineering. From sucking carbon straight out of the air to physically blocking out sunlight, the options may seem far-fetched. Can geoengineering really work? How much would it cost? And what are the risks of engineering Earth's climate?