Thermopylae (Ancient and Katharevousa Greek Θερμοπύλαι, Demotic Θερμοπύλες: "hot gateway") is a location in Greece where a narrow coastal passage existed in antiquity. It derives its name from several natural hot water springs.
Thermopylae is located in eastern central Greece on the only land route large enough to bear any significant traffic between Lokris and Thessaly. Passage from north to south along the east coast of the Balkan peninsula requires use of the pass. Further west the way is blocked by mountains and the Gulf of Corinth. For this reason the area has been the site of several battles.
The area is dominated by the coastal floodplain of the Spercheios River, surrounded by steeply sloping forested limestone mountains. The continuous deposition of sediment from the river and travertine deposits from the hot springs has substantially altered the landscape during the past few thousand years. The land surface on which the famous Battle of Thermopylae was fought in 480 BC is now buried under 20 m (60 ft) of material. The shoreline has retreated greatly over the centuries, again due to sedimentary deposition. The level of the Gulf of Malia was significantly higher during prehistoric times and the Spercheios was significantly shorter. The shoreline retreated by up to 2 km between 2500 BC and 480 BC but still left several extremely narrow passages between the sea and the mountains. The narrowest point on the plain, where the battle was probably fought, would have been less than 100 m (300 ft) wide. Between 480 BC and the 21st century, the shoreline retreated by as much as 9 km (5 miles) in places, eliminating the narrowest points of the pass and considerably increasing the size of the plain around the outlet of the Spercheios.
A main highway now splits the pass, with a modern-day monument of Leonidas on the east side of the highway. It is directly across the road from the hill where Simonides of Ceos's epitaph is engraved in stone at the top. Thermopylae is part of the infamous "horseshoe of Maliakos" also known as the "horseshoe of death": it is the narrowest part of the highway connecting the north and the south of Greece. It has many turns and has been the site of many vehicular accidents.
The hot springs from which the pass derives its name still exist close to the foot of the hill.