Sakugan

The underground is a dangerous place. People live in clusters called "colonies" and have to face the harsh conditions of the underground. To ensure humanity's survival, people can take on specialized jobs like "Workers" who mine ore and "Markers" who journey into the Labyrinth's environment to gather information. However, the colonies are threatened by creatures called "kaijuu" which range from small children to enormous buildings and are hostile to humans.

Memenpu is a nine-year-old graduate who has invented things to benefit her local colony. She dreams of a place with a never-ending ceiling and wants to become a Marker to explore the Labyrinth and search for this place. Despite her father's disagreement, an incident with a kaijuu starts a dangerous but exciting journey that could change humanity's course forever.