The trouble with fantastical legends from antiquity is that they change over time. Storytellers embellish the accounts to make them more exciting or more meaningful. The oldest versions of these stories may have been lost forever, but the best yarns begin with a grain of truth. Evidence of those grains of truth emerge when archaeologists discover the remains of the places where those stories were supposed to have occurred.
The King Arthur legends may have been based on a real person, although not in the form he takes in the stories. The places associated with Arthur are real, and more discoveries are made in those places all the time. The city of Troy figures heavily in Greek mythology, and has been unearthed gradually over 150 years of excavations in Turkey. The lost city of El Dorado could be hiding right under our noses. The Pool of Siloam was lost in Jerusalem for thousands of years before it was discovered in 2004. Read about these and other ancient places that were relegated to mythology until they were found, at Smithsonian.