I learn that if we use a singular countable noun then we must add an article or a determiner to it. There are a few special exceptions. Recently I have seen an article with the title: "Undersea World". The article talks about the world of undersea creatures, which is underwater and in the ocean. I'm wondering why the title doesn't use an article. Should it be 'The undersea world'? Because 'world' is a singular and countable noun. We are referring to a specific world. Also, sometimes I see some movies titles which don't have any articles. I'm really confused. Am I missing something about the grammar? Or we can leave out the article when the sentence is used as a title? Thanks for your help.