Work your way through the first half or so of the secret Dwarven Caverns stage and you'll reach an area where you can use a drill machine to access a switch that will open the pathway allowing you to advance. That switch is located in the room's lower right corner, and you may notice a door just beyond it that appears to be a dead end. A double jump will allow you to pass the door, though, and beyond it you'll find a cool Easter Egg that pays homage to Super Mario Bros.