[It's a familiar mirror, to Papyrus - one he checks on every time he passes through. L might notice it's less dusty than many, since he wipes it down whenever he visits. And there may still be a piece of paper tucked in between mirror and frame. But only Papyrus sees the decorations in the frame that remind him of home.]

Subterranean? Cavey?

[Synonyms are a great way to buy time as he thinks.]

I mean, it's not a little cramped cave, like that cave the Wilders visit. It's a whole long network of caves! Big enough for a big city. And lava fields, and waterfalls, and forests. Some of it's crowded...

And there aren't good roads for horses. Too many gaps.