This door is the one that leads to tunnel that connects this sphere to the main sphere.
Random Background Info:
Each sphere can seal itself off from the main sphere. It can even detach and "swim" away if the main sphere (or any other sphere, for that matter) becomes unstable or destroyed. It was one of the main priorities when TENka City was first proposed and being built due to the disaster of the previous underwater city. Also due to the Ending and hyped-up fear/dread for impending calamity, these outside doors are kept sealed when not in use. Most people tend to stay in their own spheres and only travel when needed. Jake is not one of these.