system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In a big city like Chicago how are the drainage and sewer systems maintained during winters, when temperatures drop to like -20 and everything just freezes instantly?
gpt
There's this thing known as a "frost line". It's the depth that the ground will freeze to, given a prolonged temperature.

Cities know how cold it can get, and bury their sewage lines well below the frost line. This ensures that they don't freeze, even in extremely cold winters.