system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do the underground pipes that deliver water for us to bathe and drink stay clean? Is there no buildup or germs inside of them?

Without any regard to the SOURCE of the water, how does water travel through metal pipes that live under ground, or in our walls, for years without picking up all kinds of bacteria, deposits or other unwanted foreign substances?  I expect that it's a very large system and not every inch is realistically maintained and manually cleaned.  How does it not develop unsafe qualities?
gpt
The water supply contains sanitizers once it has been treated for use. If you have a sealed system that you're constantly pumping sanitized water through, it's unlikely to get contaminated barring some failure of the sanitation or the piping itself.