I possibly could have found a better word for U, but it was unlikely. As it is, I really don't have much to say.
A small portion of the book takes place underground. First they go to the Old Woman's cave, which has low ceilings and Jen declares them completely uncomfortable.
Later, Jen and Tisha hide in a random cave whilst Chris is fighting a dragon. During this time, the two have a deep and heartfelt (cough, cough) conversation about physical appearance and rules. I didn't describe this cave.
And then Chris and Tisha hide in the Labyrinth after they lose Jen and think her dead. (Don't ask me why they thought that. They weren't thinking clearly, I guess) The Labyrinth is one of my favorite parts of Jen's imagination, and it will get a short story eventually. It may even factor into the first one I plan to release ("The Sun Jewel Adventure").
But after that, they make the playground their home base, and we don't go into any more caves (well, Jen goes back to the Old Woman's, but that doesn't count.)
So ... There's U. Short, sweet and ... U.