Treasure Isle: Fire God Mountain takes players to the Center of the Earth
To reach Fire God Mountain, you need to travel to the first page of Tiki Isles, where you'll spot a burning, animated volcano in the north. To get in, you're going to need 5 tubes of SPF-3000. To note, you'll start with 6 tubes for free, but it won't be enough on the long dig down, so you've gotta use Wall posts (those tubes aren't giftable) to request more from fellow players.
For more info, check out the "Volcano FAQ" tab on your Treasure Island window, and the newly updated Fire God Mountain Info Thread at the Zynga forums. Click here to launch the game and get digging!