From Bulbapedia, the community-driven Pokémon encyclopedia.
The Hill of the Ancients (Japanese: せいれいのおか Hill of Spirits) is a location in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team. It is on top of the Great Canyon. Xatu resides here, looking into the past, present, and future by staring at the sun with unblinking eyes.
Xatu is able to tell that the player is actually a human, transformed into a Pokémon. He is also able to communicate with Pokémon anywhere by using telepathy. He predicts the coming of a meteor that will collide with the world, destroying it. He creates the Teleport Gem with the aid of Alakazam and Gengar, so that the player's team can travel to the Sky Tower in order to ask for Rayquaza's help to prevent the meteor's collision. Some time afterwards, he tells the player of a "mirage Pokémon" and directs them to the Fiery Field.
In the manga
Hill of the Ancients in Pokémon Mystery Dungeon: Ginji's Rescue Team
In the Pokémon Mystery Dungeon: Ginji's Rescue Team manga
Ginji, Mudkip, and Gengar visited the Hill of the Ancients in I Want to be Human Again!. At first, Xatu didn't pay any attention at them, being focused on looking at the Sun. He didn't even flinch when he was hit by Ginji's Flamethrower, but soon he came back to his senses and yelled in pain for being burned. After this had been dealt with, Mudkip asked Xatu to look into Ginji's future, not noticing that Gengar had come with them just to steal stuff. Xatu looked into Ginji's future and saw that he was actually a human. He also predicted a dark future, not being able to provide further details at the time.