I didn't know it needed one.

(Seriously, it's news to Luffy and he can't help but wonder about it now. And what, traveling through the sky??

Luffy absently scratches the back of his head.)

We have the Red Line and the Grand Line, oh and the oceans East Blue, West Blue, North Blue, South Blue. Not sure about a name for everything, but my friend Nami would know. Traveling through the sky is mostly birds, though I can manage it if I throw myself hard enough?

You're not wrong though there is a lot of space up there.