Significance of The Sky/Cavern roof

The sky marks the cave where Atlas had been imprisoned, forced to hold up the cavern ceiling, effectively holding the weight of the world on his shoulders. The sky represents a cruelty that has been explored with the gods. To someone like Luke, it serves as motivation to want to overthrow the gods, since it shows how callous they can be towards the people underneath them. Even to characters like Percy, it serves as a reminder that the gods aren’t the most goodly or perfectly moral beings in the world. This is also where the book gets the title, as the Titan’s curse is reference to Atlas’s punishment having to hold up the sky.