What is That?!

What is that? I remember thinking, as I caught a glimpse of a giant elevated grate in my daughter's Toronto neighborhood. 

Was that an outdoor art fixture? Or did it serve some practical purpose - like covering a very large water reservoir?

Let's go check it out, Terry suggested - so we did.

We discovered that it covered two commuter train tunnels that extended below street level for several blocks.

Were it built for aesthetics? Or did it serve a practical purpose? I wondered...

When I googled my question, I saw that I wasn't the first person to wonder about this...

They were needed to support the walls of the train tunnel - to prevent them from caving in. (I'm glad they look nice, as well!)