Multiple Dimensions!

You will need to craft the Wizard of Space and Time building. The recipe to craft the building will be earned after the second boss, Gath'ack the Soulgazer, is killed. Wizard of Space and Time Recipe:
 * 30 Shaped Stone Bricks
 * 10 Wood

After crafting the tower talk to the Wizard NPC inside who sells an item called 'world books'. Read the book and every layer under the Muddy Layer will be reset allowing you to fight bosses multiple times or clear the whole map looking for portals, shrines, or other items.