 In 1816 , the couple famously spent a summer with Lord Byron , John William Polidori , and Claire Clairmont near Geneva , Switzerland , where Mary conceived the idea for her novel Frankenstein . The Shelleys left Britain in 1818 for Italy , where their second and third children died before Mary Shelley gave birth to her last and only surviving child , Percy Florence Shelley . In 1822 , her husband drowned when his sailing boat sank during a storm near Viareggio . A year later , Mary Shelley returned to England and from then on devoted herself to the upbringing of her son and a career as a professional author . The last decade of her life was dogged by illness , probably caused by the brain tumour that was to kill her at the age of 53 . 
