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.

The caverns of Bism where presumably flooded and darkened when the world came to an end. Since the Dragons and Salamanders made their way to the surface, via newly created tunnels, consumed the vegetation, died and rotted into skeleton structures, the Gnomes presumably made their way through these tunnels to the Overworld and entered Aslan's Country through the stable door. Once Father Time had destroyed the Moon-Sun, the caverns, along with the rest of the new body of water became solid ice.