Mountains' Graveyard

The is a mountainous region located at the north-most region of land between Takigakure and Otogakure. It has dense forestry and the bones of many giant creatures can be seen jutting out of it. It contains an abandoned mine that is connected to Ōtsutsuki Clan ruins, which Madara and Obito Uchiha used as their secret hideout for the duration of the Eye of the Moon Plan.

At the start of the Fourth Shinobi World War, Kabuto Yakushi managed to locate the place while also using a trail of corpses to lead Anko Mitarashi's team there as well.

Overview
Some time after being defeated by Hashirama Senju, Madara set up a base here. As Madara was living off life support provided by the Demonic Statue of the Outer Path, White Zetsu clones were instead used to gather information from the surface world and carry out any other functions as they could travel through the earth. There were passages that seemingly led to different countries as Madara was able to have the White Zetsu retrieve a heavily wounded Obito who was in the Land of Grass. This passage was eventually closed off on Madara's orders and later smashed open by Obito with the powers of the Demonic Statue of the Outer Path. A part of the cave was destroyed by Sasuke when he tested out his Eternal Mangekyō Sharingan, while another part of the hideout was damaged when he escaped.

When it was occupied by Madara, there was only Hashirama's living clone and the Zetsu minions released by Black Zetsu, the tree which grew from it and the Demonic Statue seated upon it, a large tree stump which Madara occupied, and a wooden bed on a single platform. On a wall hung several of Madara's tools such as his gunbai, a kama, a chain with weights, several swords, giant shuriken and some spare cloaks. Though Madara claimed there was no exits where they were, many hidden entrances and exits had been carved out, concealed beneath the bones of animals, or else etched into the walls. Shortly before the Fourth Shinobi World War, beneath the clone of Hashirama is a flooded cavern where the rest of the White Zetsu Army clones were stored. Fifteen years following the Fourth Shinobi World War, having lost the Demonic Statue as it's chakra supply, the flowering tree withered, with the water beneath it having seemingly dried up. Surviving remnants of the White Zetsu hid themselves among the roots of the tree, while the still-developing Zetsu attached to it decomposed. A majority of them were eventually discovered and killed by Sasuke.

Laboratory
Within the base there is also a laboratory where Obito carries out various experiments. Obito brought Danzō Shimura's corpse here in hopes of retrieving Shisui Uchiha's Sharingan only to discover that the elder had crushed it before his death — an action which Obito cursed him for. He also transplanted Itachi Uchiha's Sharingan into Sasuke here.

Inside the lab is a massive wall decorated with the Uchiha clan symbol on top of it, which stores a large collection of eyes, which are each individually suspended in fluid inside built-in labelled glass containers, allowing Obito access to many Sharingan replacements should he need them. The laboratory is also equipped with an operating table and instruments for surgical use, such as his attempt to extract Shisui's eye and transplant Itachi's eyes into Sasuke.

Trivia

 * In an episode summary for Boruto: Naruto Next Generations episode 51, the mining town that is in proximity of the Ōtsutsuki Clan ruins is said to be near the border of the Land of Fire.