Not any sidewalk

Stand next to the old chair factory. Take a look around and then look down. It's all solid matter, but this matter doesn't have matter underneath it. The matter is a sidewalk, and it's metal, and it's hollow. You're standing there, but there's an abyss below.

What is down there? Utility basement? Vice President? Real estate agents?

Don't be too heavy, and don't jump too hard, and don't be a truck and drive here. Don't do it, it's a hollow sidewalk.