Tiny, transparent, and threatened, krill are crucial to the Antarctic ecosystem. But the population of krill is crashing for reasons that continue to baffle the experts. A leading theory says that krill's life cycle is driven by an internal body clock that responds to the waxing and waning of the Antarctic ice pack, and as climate change alters the timing of the ice pack, their life cycle is disrupted. To test it, NOVA travels on the Polarstern, a state-of-the-art research vessel, to the frigid ice pack in the dead of winter. From camps established on the ice, scientists dive beneath the surface in search of the ice caves that shelter juvenile krill during the winter. There, they hope to discover what's causing the krill to vanish and, ultimately, how the shifting seasons caused by climate change could disrupt ecosystems around the world.

On a summer's night, there's nothing more magic than watching the soft glow of fireflies switching on and off. Few other life forms on land can light up the night, but in the dark depths of the oceans, it's a different story: nearly 90% of all species shine from within. Whether it's to scare off predators, fish for prey, or lure a mate, the language of light is everywhere in the ocean depths, and scientists are finally starting to decode it. NOVA and National Geographic take a dazzling dive to this hidden undersea world where most creatures flash, sparkle, shimmer, or simply glow. Join deep sea scientists who investigate these stunning displays and discover surprising ways to harness nature's light—from tracking cancer cells to detecting pollution, lighting up cities, and even illuminating the inner workings of our brains.