Pellucidar

Could this be the lost world Edgar Rice Burroughs wrote about

-Elsa after she entered pellucidarPellucidar is a vast perfectly preserved mesozoic jungle located underground

Lore
Pellucidar had long existed as a miner's legend passed down by word of mouth and in hastily drawn sketches. No one believed the fantastic tales of prehistoric creatures still roaming the earth. Pellucidar was a complete geological and evolutionary anomaly, a place of inexhaustible mysteries. But when five residents of the kingdom Arendelle and a Guardian of Childhood first arrived in Pellucidar they found themselves running from giant beasts that up to that point had only been known from the fossil record. Before anyone had discovered Pellucidar it was common knowledge that dinosaurs had been extinct for 65 million years. The prevailing theory has held that an asteroid collided with the earth 65 million years ago fermenting a prolonged global winter. Dinosaurs and fifty percent of all animal species became extinct. But in a massive underground cavern that theory runs into gargantuan obstacles.

Background
The translucent quarts ceiling is supported by rocky pillars that stand as high as skyscrapers. There are waterfalls and forests of cycads that were similar to the appearance of pine cones and palm trees, which were common in the Mesozoic Era. The light coming down on the jungle was caused by the sun that shone through the thick layers of gemstone up above. In other words, it was as bright as day. It is also very warm, like a mild rainforest. Apparently this is where dinosaurs and other Mesozoic inhabitants escaped to in order to survive the K-T Event. Despite all the changes that had perfected the rest of the world a menagerie of creatures had survived intact. Pellucidar's most common inhabitants are dinosaurs (from the Jurassic and Cretaceous periods), though pterosaurs and giant insects have also been shown to be native here.

Pellucidar is a fictional Hollow Earth invented by Tarzan creator Edgar Rice Burroughs for a series of action adventure stories. In a notable crossover event between Burroughs' series, there is a Tarzan story in which the Ape Man travels into Pellucidar.