system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How were subway systems built in cities like NYC, Rome, etc. without the buildings above them collapsing?
gpt
Massive amounts of engineering, and braces are installed in the tunnels as they are being bored out. Usually big reinforced-concrete casings are added to each successive length of tunnel. 

There is a lot that goes into a subway system — the tunnels usually have to allow for drainage, and they have to account for the groundwater levels, as well. It complicates things when the groundwater level isn’t below the tunnel.