Why does my main water supply go down into the foundation?
This is very common, but not good. It's not good because it's under the concrete slab &amp; expensive to get to, as well, these do usually fail very prematurely &amp; first (30 to 60-years) because they live in harsh &amp; moist conditions to be constantly under attack.

But, this is "normal" for basement baths &amp; laundry sinks or laundry supply hook-ups. It was just cheaper, easier &amp; shorter to run the pipes this way. A lazy Plumber that didn't care about the future &amp; a crap builder that went along with it...just like today.

If you're saddled with a leak in the future, that's when you should see if you can run that piping up into walls, across ceilings &amp; back down walls instead...unless you're doing renovations that can correct it sooner. This will double the piping's lifespan &amp; be easily repaired in the future.