In time technology advances. We will be able to dig wider and wider. Imagine parallel, multi-level 3d interconnected framework built up from tunnels. The possibilities are endless. A sketch to show the main idea. We’ll zoom into them on the next page. For now, just notice the scale. Within relatively small openings we can fit in “skyscrapers” or a forest.
Most agricultural activities, transportation, lifts could fit within smaller, cheaper tunnels. Complex social activities would fit in wider ones. Interconnecting them would make it feel more open. Even more open than walking in a crowded city (here on Earth). Point to point connections would let us avoid key points and rush hours. The whole … million-person city would feel more like a “mountain village” or a “deserted island”. Decentralizing traffic and infrastructure would keep the perceived density low.