I’m just gonna get rid of that first deep down, if it’s okay with you. I’m also deleting the stairs after the second down, because the stairs were already mentioned, so how else would you be going down?
Use italics for emphasis, not ALL CAPITAL LETTERS. 🙂
In the forest, you discover a cavern with a perfectly rectangular tunnel and stairs going deep into the earth. As you go down, you find a city, a gigantic city.
Why didn’t I capitalize earth this time? Because it refers to the ground, not the planet.