Okay kiddo, imagine you're walking down a busy street, and you see all the cars and buses zooming by above your head. That's what we call an overpass - it's a way for cars and other vehicles to go over something, like a road or a river or a building.
Now, what if you wanted to go under that busy street, instead of over it? That's where Park Street Under comes in. It's a big tunnel that was built underneath Park Street in Boston, Massachusetts, so that people could walk or drive under the busy street, instead of having to cross it.
Inside Park Street Under, it's dark and kind of spooky, but there are lights to help you see where you're going. There are also shops and stores down there, so you can do some shopping while avoiding all the traffic up above.
Park Street Under is kind of like a secret underground world, where you can stay safe and avoid all the noise and chaos of the street above. It's a pretty cool place to check out if you ever get the chance!