The underground cavern is a cave buried deep within the Earth's crust visited by Beast Boy on his quest to retrieve the King's Lettuce to recreate the Legendary Sandwich.
After being entranced by the Legend of the Legendary Sandwich, Beast Boy volunteered to retrieve the King's Lettuce due to his ability to transform into a mole, an animal capable of burrowing underground. He sniffed out the ingredient but was challenged by a Sandwich Guardian before he could open the chest. After a brutal and long fight at the cost of his teeth, Beast Boy crushed the Sandwich Guardian by transforming into a whale. Upon retrieving the lettuce, he returned to the surface.