What circumstances in the summer of 1816 prompted Mary Shelley to write this novel?

In the summer of 1816, Mary Shelley visited Geneva Lake (Switzerland) and spent her days in Switzerland with Lord Byron, Percy Shelley, and Dr. John Polidori. One day, after reading a collection of German short stories, called Fantasmagoriana, they decided to write their own short, supernatural stories.  

Her story was called Frankenstein. Mary Shelley decided to convert her story into a novel, so she continued to write it until she finally completed the novel in 1817. The novel was first published anonymously, in 1818.

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and also the mystery girl it might be someone that she might know in her school