Not long after the civilization has moved to the Underwater City, the climate changes precipitated by the Last War started affecting the seas as well. Large sea monsters, thought to exist only in legends have started to appear regularly, patrolling the seas and making all naval traffic all but impossible. While the appearance of these creatures further contributed to the seclusion of Underwater City from the rest of Planet Banimon, it did not affect their food or energy supply, both of which are heavily dependent on the sea.
True to their isolationist tendencies, the people of Underwater City decided to let proverbial monsters lie. These survivors of the Last War remain forever marked by that horrid experience, and the fear of bringing about another desaster has guided their actions ever since. Satisfied that their substenance has not been endangered, they put a stop to any additional underwater exploration for fear of antagonizing these mighty creatures, turning further inward.
As a result, the seas have remained largely unexplored, begging the question of what else might remain hidden under the waves.