"It is not this world, but the next, in which our true glory lies."- LSA leader Salvador Limones to Special Agent Manny Calavera, regarding the opening of Calavera Café and settling down in the Land of the Dead.
The Ninth Underworld (also known as The Land of Eternal Rest) is the ultimate goal and final destination for all those in the Land of the Dead who go on the Four-Year Journey of the soul. It is accessible only via a train tunnel within an enormous, Mayan-style temple in the middle of a vast, arctic wasteland. The tunnel can either be entered via train, or simply walked through.
The Ninth Underworld is never actually seen in-game, nor is it ever described with anything other than vague terms. It can be assumed, however, to be a glorious place where weary souls can rest forever.
If Manny interacts with the train tunnel, he remarks on how he can easily drop everything and go forth to the Ninth Underworld right then and there, but chooses not to as he feels responsible for all the other people who still need his help and unraveling the counterfeit ticket conspiracy.