The images below, taken from Google Earth, show the site of the Battle of Thermopylae as it looks today. The upper image, taken at an oblique angle, gives a sense of the terrain. Steep hills on the south side of the modern road mark one side of the narrow pass. On the other side, I’ve marked the ancient shoreline, now inland. Leonidas’ statue sits at the approximate site of the battle.
The image below, taken from directly overhead, shows the large land area created from sediment deposited since 480 B.C. The coastline of the Gulf of Milia is now a couple of miles north of its earlier location.