The stairs kept going further and further down underneath the earth. Underneath the city until natural light was no longer able to filter in.
Where is your character headed? Why is he/she going there? Remember the novel City of Ember? In the novel everyone lived underground for generations because “outside” wasn’t safe. Then one day the world under the earth started to fall apart and they are outside into a new world.
What’s your story? Strange places and unique situations help to make stories interesting. I think this is a great writing prompt for a teen. Be creative and have some fun!