Where is the ground in Chicago?

[faraway]

I noticed in Chicago several streets where it looks like the street is at a higher level than the ground. Like on a levy. Most houses reach the sidewalk at level, some with a little bridge and some with a few steps. If you look between the houses you can see the ground is below street level. Surprisingly is the ground back to street level when you go to the next intersection.
Can a Chicago resident shed some light on that surprising effect?

This is just one example. Peek on both sides of the street between the houses.
https://maps.google.de/maps?q=chicago+il...54,,0,4.92