This will unlock once you've reached the very top of the world. You'll know you're close when you see stars and gravity weakens. You can also check your map.

There are a few ways to do this. Using a Gravitation Potion is the easiest. You can also get there by making a "dirt pillar", manually pointing the cursor below you and jumping straight up, while placing blocks below you as you're in the air. Because of Harpies, it is best to wait until you have a Grappling Hook so you can recover quickly after getting attacked.