The title refers to the distance the _Nautilus_ traveled while submerged, not the depth it reached.

Until I realized this, the title was quite confusing. If “20,000 leagues” were referring to depth, it would be enough to go all the way through the Earth, exit the other side, and then make it a quarter of the way to the moon.

Yeah he really needed a comma.

20,000 Leagues, Under the Sea

I think it reads cleaner.