Bism

Bism was the nation/state six thousand feet beneath Underland at the bottom of the World of Narnia, also called the Really Deep Realm. It was the home of all Narnian Gnomes where crystals, diamonds, and gold grew and thrived in a similar way to plants and whose "juices" could be harvested for food and drink. Once the gemstones died, they fossilised and became the hard gemstones that humans are used to collecting on the surface of the earth. Bism was also full of great lava rivers that were inhabited by Salamanders and Dragons; Bism was nearly visited by Prince Rilian during the Fall of Underland.

''When they got used to it they thought they could make out a river of fire, and, on the banks of that river, what seemed to be fields and groves of an unbearable, hot brilliance—though they were dim compared witht he river. There were blues, reds, greens, and whites all jumbled together: a very good stained-glass window with the tropical sun staring straight through it at mid-day might have something the same effect. - From The Silver Chair'', Chapter Fourteen: The Bottom of the World

Jill, Eustace, Puddleglum and Prince Rilian were attempting to escape from Underland when they encountered a gnome named Golg who told them about the land of Bism. Rilian initially wanted to visit the land of Bism because his father Caspian X explored lands to the end of the world and he thought it would be fitting if he explored lands to the bottom of it. However, the four could hear a Salamander speak to them from far below, saying "the chasm, it closes!" A few minutes later the entrance to Bism had slammed on itself.

Bism could have survived the end of the world because it was underneath the creatures that ate the world and it was under the great flood. So it is possible that it survived the end of the world or if it did end too some of the creatures could have came down to Bism and ate it too. Though it is unknown how the Gnomes would make it to the door to Aslan's Country as there is no known passage from Bism to Overland.