Okay kiddo, so a long time ago, Seattle was a really small city and it didn't have the big buildings and roads that it has now. But as more people came to live in Seattle, the city started to get bigger and bigger, and they needed to make new roads and buildings.
But in order to build the big roads and buildings, they had to move a lot of dirt and rocks, and that made the land higher than the sidewalks and buildings. So the sidewalks and buildings were like a big staircase, and people had to climb up and down to walk around the city.
This was not very convenient, especially when it rained a lot in Seattle, which it often does. So the city decided to build everything up one level higher. They made new streets and sidewalks and buildings on top of the old ones, so people could walk around without going up and down stairs all the time.
But that left the old streets and sidewalks and buildings down below, and nobody wanted to use them anymore because they were too low and it was too hard to get down there. So the old streets and sidewalks and buildings became like a secret world, hidden underneath the new city.
Now people call this secret world the Seattle Underground. It's like a secret tunnel or maze that's underground, and you can only go there on a tour. When you go on a tour of the Seattle Underground, you can see the old sidewalks, buildings, and streets that were used a long time ago, and you can even learn about the history of Seattle while you're down there.
Isn't that cool, kiddo? The Seattle Underground is like a secret world that's hidden under the city!