Keyhole

A Keyhole is a magical gateway in each world that leads to the Heart of that world. The Heartless invade worlds and search for their Keyholes (which, not surprisingly, appear as large keyholes). Upon discovering the Keyhole, the Heartless enter it and devour the world's heart. After a world's heart is consumed, the world crumbles, and fades from existence. It is say that, if the Keyblade locks a world's keyhole, it can never be opened again.

Kingdom Hearts
The Heartless invaded Destiny Islands, the home of Sora, Riku and Kairi. Not only did they attack the islands in search of the Keyhole, but also to find the Keyblade Master and one of the Seven Princesses of Heart, who is Kairi. The islands were eventually destroyed when the Heartless attacked Kairi and managed to get into the Keyhole (located in the Secret Cave). Sora ended up in Traverse Town by going through a Corridor of Darkness (which opened during the islands destruction), while Riku ended up in Hollow Bastion. Kairi's body was found by Maleficent. In Traverse Town, Sora learned from Leon and Yuffie that the Heartless took over their world by order of Maleficent. Sora, along with Donald and Goofy, decide it is their duty to travel from world to world and seal the Keyholes using the Keyblade.

Maleficent and her gang of villains are completing a artificial Keyhole in Hollow Bastion's by obtaining the seven hearts of the Princesses of Heart, that Keyhole, once completed, would lead them into Kingdom Hearts, the heart of all kingdom's and source for unmeasurable power and knowledge. However, without Kairi's heart the Keyhole remained incomplete. Her hearts was actually dwelling inside Sora's. Sora eventually releases Kairi's heart, completing the Keyhole. Ansem, who is possessing Riku's body, ventures into the Keyhole in order to get to Kingdom Hearts. Sora seals the Keyhole and then follows Ansem to the End of the World, a place formed from the remains of world's taken by the heartless. Deep in the darkness of that place lies the Door to Darkness, with Kingdom Hearts waiting on the other side. There, Sora battles Ansem. Ansem them opens the door so he can obtain the darkness inside Kingdom Hearts, but Sora calls out the Light from inside the door and Ansem is destroyed. Sora, Riku and King Mickey sealed away Kingdom Hearts by locking the Door of Darkness on both sides sealing it as well. All of the destroyed worlds are restored, including Destiny Islands. It is presumed all the restored worlds had their Keyhole sealed automatically and hidden, so they could not be destroyed again.

World Keyhole Locations

 * Destiny Islands: The Keyhole takes the form of a door in the Secret Cave. It was opened by Riku.
 * Traverse Town: Traverse Town's Keyhole is located in the water fountain in the Second District.
 * Wonderland: The Keyhole is appropriately hidden inside the mouth of the Doorknob.
 * Deep Jungle: The Keyhole is hidden at the top of the waterfall rock face in a small cave, behind a flock of butterflies.
 * Olympus Coliseum: The Keyhole was hidden under a stone block in the coliseum's lobby.
 * Agrabah: The Keyhole is in the final room of the Cave of Wonders. After stealing the lamp from Aladdin, Jafar's first wish was revealing it.
 * Atlantica: The Keyhole is hidden in Ariel's Grotto. After defeating Ursula, Triton allowed Sora to lock the Keyhole using the Trident to reveal it.
 * Halloween Town: The Keyhole was hidden under the house of Oogie Boogie. After it collapsed, the moon reflected on the spot where the Keyhole was.
 * Neverland: Neverland's Keyhole is located on the clock face of Big Ben.
 * Hollow Bastion: The Keyhole to this world has some controversies around it. On the Ansem Reports, Xehanort states that he found a strange door in the basement of the castle, the door had a large keyhole on it and it wasn't locked, so he opened it, seeing on the other side what appeared to be the world's heart. However, Maleficent was gathering the princesses to open the "Final Keyhole" on the top of the castle, where such Keyhole was located.  So it appears that the Keyhole found by Xehanort in the basement was the actual Keyhole to that world, while the Keyhole Maleficent was working so hard to create was an artificial Keyhole to be used as a pathway to Kingdom Hearts.
 * 100 Acre Wood: The Keyhole is on the clasp that closes the book.