This illustration depicts a fictional city design, established on the recently discovered exoplanet TRAPPIST-1d by the crew of a failed mission and its descendents. It shows a cross-section of the cave system in which the main city is located, along with details of the city and native plant life along the borders. At the top is the historical district, which preserves what remains of the original settlement established by the crew of the mission. The libraries, museums, and public service buildings are located here. At the lower left is the botanical district, established later when the inhabitants established building methods that could withstand the high winds of the exposed area within the arch. This is where Earth plants are grown from seeds brought with the original crew, inside of large indoor facilities where the climate and lighting conditions can be reliably controlled. The city proper is built inside of a man-made cavern in the center. This is where the majority of the citizens live, as well as the location of most leisure activities. To the far right, a secret settlement exists within a largely otherwise unexplored cave system. This settlement was created by citizens who became dissatisfied with their place in the city's societal structure. The cave is also populated by bioluminescent creatures, whose properties then are transferred to those who live around them and ingest them. Because of this luminous quality, those who live in this cave are treated as a sort of urban legend by those in the city proper; their presence in the public mind is somewhat like that of ghosts or other supernatural beings.