The Lost Underworld is an area in EarthBound that can be reached after Ness and co. leave Lumine Hall, and leads into the eighth and final Your Sanctuary location, Fire Spring. The overworld of this place is mostly a big landscape with very small sprites, and the enemies are mostly dinosaurs.
The area is first referenced in Fourside, when the player talks to Mr. Spoon in the dinosaur museum. After talking with Ness and the party for a while, he pulls them aside and informs them of rumors stating that dinosaurs still exist near southern Scaraba.
When Ness and co. finally get there, they find that the entrance is blocked by a giant rock. After leaving the Tenda village, Apple Kid calls Ness and co. with information concerning a new invention, the Eraser Eraser, as well as information about Ness's main enemy, Giygas. However, disaster strikes when Apple Kid is kidnapped by Giygas's minions. After freeing Apple Kid and others from the Stonehenge Base, Ness learns that Apple Kid returned the book 'Overcoming Shyness' to the Onett library. After obtaining the book and lending it to the leader of the Tendas, a very strong Tenda lifts up the rock that was blocking the way to Lumine Hall. Ness goes on to complete the dungeon and defeat the Electro Specter, allowing Ness to gain access to the Lost Underworld.
Following a brief cutscene, Ness and co. drop down a hole and find themselves in the Lost Underworld. The area is populated by Wetnosaurs, Chomposaurs and Ego Orbs. The place also sports hot springs, another village of talkative Tendas, and another talking rock that tells Ness of the eight Your Sanctuary locations. The place is also the location of the Cloak of kings, one of Poo's only equipable items. There are two kinds of hot springs. The kinds with blue water restore HP and PP, while the ones with red water heal certain status effects.
On the north-west of the underworld, there is a cave entrance that leads you to the present version of the Cave of the Past called "The Cliff that Time Forgot" because the cliffs are floating on nothing. You can't explore the whole cave because some parts of the cliffs are broken and missing so it's not necessary to go there. On the south-west of the underworld, there is a cave entrance that leads you to the last Your Sanctuary location: it's the Fire Spring sanctuary.