Volcano, is the "boss world" of Xio. It contains only two levels, Volcano Gate, and Volcano Depths. You must get the Key Cards from both The Mine, and The Skyline to access it, although you CAN enter the world before getting them. If you do, you are simply presented with a room with two doors, locked if you haven't beat the bosses from The Mine and The Skyline. Volcano is the 5th world of Xio, after the Dark Forest, Outskirts, Mine, and Skyline.