From Bulbapedia, the community-driven Pokémon encyclopedia.
The Summit Ruins (Japanese: 山の遺跡 Mountain Ruins) are ruins that are located near the Pokémon Summer Academy. It was featured in Ghoul Daze!. One of the activities of the academy included the students pairing off in teams and retrieving a medal hidden in the ruins.
The ruins contained a cave which was believed to be the entrance to the spirit world. It was uncovered during a construction accident. A ghost girl from the spirit world tried to lure people into the portal but was fortunately prevented from doing so by a wild Dusknoir. After it pushed the ghost back into the portal, the cave collapsed, sealing the portal.
Here is a list of all the Ghost-type Pokémon that reside in the forest around the ruins.