Final answer:
The author's purpose is to entertain the reader with a story about aliens trying to destroy Earth and people trying to fight back.
Step-by-step explanation:
The author's purpose in this passage seems to be to entertain the reader. The story is about aliens trying to destroy Earth and people fighting against them. The author wants the audience to be engaged and entertained by the action-packed plot and the idea of a underground city as a possible solution.