The midnight zone is the deepest part in in ocean, a place where even experts don't know much about. There is still a lot of the midnight zone to explore. This abyssal zone is impossible to see in without a light, as the sun can never shine down there. Yeti crabs, siphonophores, anglerfish, hatchetfish, jellyfish, tubeworms, ventfish, worms, are some of examples that the Octonauts have met down there. Yet more exists down there. The midnight zone is the last layer of the three zones that are featured in Octonauts, the sunlight zone, the twilight zone, and then the Midnight zone.
There is a deeper layer of the ocean, the Abyssal zone, yet it has never been featured in Octonauts.
It would really help to have a picture here!