system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If all of Manhattan is basically hollowed out underground for the subway systems, what prevents all the buildings from collapsing in on its self?
gpt
Tunnels in cross section are basically arches with the brick or concrete lining acting as the pillars.  That leads the stress on the top down to the ground beneath.  They will design the tunnels and the routes they take to support the weight above and would avoid hollowing out beneath a building without carefully calculating the stresses they need to account for.  Often tunnels will take routes that don't have high loads above, like following the line of a road.