Task: Question Answering
Input (English): Passage: While flying a routine mission for the U.S. Navy from his aircraft carrier, an emergency causes Lieutenant Robin "Rob" Crusoe to eject from his F-8 Crusader into the ocean. Crusoe drifts on the ocean in an emergency life raft for several days and nights until landing on an uninhabited island. Crusoe builds a shelter for himself, fashions new clothing out of available materials, and begins to scout the island, discovering an abandoned Japanese submarine from World War II. Scouring the submarine, Crusoe also discovers a NASA astrochimp named Floyd, played by Dinky.Using tools and blueprints found in the submarine, Crusoe and Floyd construct a Japanese pavilion, a golf course, and a mail delivery system for sending bottles containing missives to his fiancee out to sea.
Soon after, Crusoe finds that the island is not entirely uninhabited when he encounters a beautiful island girl, whom he names Wednesday. Wednesday recounts that due to her unwillingness to marry, her chieftain father, Tanamashuhi, plans to sacrifice her and her sisters to Kaboona, an immense effigy on the island with whom he pretends to communicate.
The day Tanamashu arrives on the island, Crusoe uses paraphernalia from the submarine to combat him, culminating in the destruction of the Kaboona statue.
After the battle, Crusoe and Tanamashu make peace. But when Crusoe makes it known that he does not wish to marry Wednesday, he is forced to flee to avoid her wrath. Pursued by a mob of irate island women, Crusoe is spotted by a U.S. Navy helicopter and he and Floyd narrowly escape with their lives. Large crowds turn out for their arrival on an aircraft carrier deck, but Floyd steals all the limelight. 
Question: What did Robin and the astrochimp build that makes it possible for Robin to communicate with his fiancee?
Output (English): a mail delivery system.