Thermopylae (//; Ancient Greek and Katharevousa: Θερμοπύλαι (Thermopylai) [tʰermopýlai], Demotic Greek (Greek): Θερμοπύλες, (Thermopyles) [θermoˈpiles]; "hot gates") is a place in Greece where a narrow coastal passage existed in antiquity. It derives its name from its hot sulphur springs. The Hot Gates is "the place of hot springs" and in Greek mythology it is the cavernous entrances to Hades.
Thermopylae is world-famous for the battle that took place there between the Greek forces (notably the Spartans, Lachedemonians, Thebans and Thespians) and the invading Persian forces, commemorated by Simonides in the famous epitaph, "Go tell the Spartans, stranger passing by, That here obedient to their laws we lie." Thermopylae is the only land route large enough to bear any significant traffic between Lokris and Thessaly. This passage from north to south along the east coast of the Balkan peninsula requires use of the pass and for this reason Thermopylae has been the site of several battles.
In ancient times it was called Malis which was named after the Malians (Ancient Greek: Μαλιεῖς), a Greek tribe that lived near present-day Lamia at the delta of the river, Spercheios in Greece. The Malian Gulf is also named after them. In the western valley of the Spercheios their land was adjacent to the Aenianes. Their main town was named Trachis. In the town of Anthela, the Malians had an important temple of Demeter, an early center of the Anthelan Amphictiony.
The land is dominated by the coastal floodplain of the Spercheios River and is surrounded by sloping forested limestone mountains. There is continuous deposition of sediment from the river and travertine deposits from the hot springs which 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 metres (66 ft) of soil. The shoreline has also advanced over the centuries because of the sedimentary deposition. The level of the Malian Gulf was also significantly higher during prehistoric times and the Spercheios River was significantly shorter. Its shoreline advanced by up to 2 kilometers between 2500 BC and 480 BC but still has left several extremely narrow passages between the sea and the mountains. The narrowest point on the plain, where the Battle of Thermopylae was probably fought, would have been less than 100 metres (330 ft) wide. Between 480 BC and the 21st century, the shoreline advanced by as much as 9 km (5.6 mi) 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 to King Leonidas I of Sparta on the east side of the highway. It is directly across the road from the hill where Simonides of Ceos's epitaph to the fallen 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 exists close to the foot of the hill.