The springs that gave the pass its name still bubble at the foot of Mount Kallidromo. Hot sulfur water, around 40 degrees Celsius, fed by the same fault systems that thrust up the limestone mountains, flows into pools that have been used for bathing since prehistory. According to one version of the labors of Heracles, the waters became hot when the hero washed himself there to cleanse the venom of the Lernaean Hydra. Greek mythology held the Hot Gates as one of the entrances to Hades, the underworld. The connection was not metaphor alone. The first known Amphictyony, an ancient league of religiously associated tribes, gathered at the nearby town of Anthela around the cult of Demeter, who in her older forms was a chthonic deity tied to the underworld. The delegates were called Pylagorai, gate-assemblers. The gates they assembled at were these.

The pass sits at the base of a horseshoe of mountains that drops sharply toward the Malian Gulf, with the floodplain of the Spercheios river extending east. Two processes have been steadily filling the gulf for thousands of years. First, the Spercheios carries sediment off the central Greek mountains and dumps it at its delta. Second, the hot springs deposit travertine, a calcium carbonate that builds up around the discharge points. Between roughly 2500 BC and 480 BC, the shoreline advanced by up to two kilometers. After the famous battle, deposition continued. The Greek geologist Spyridon Marinatos and later geomorphologists have mapped the shoreline at different periods, and the picture is dramatic. Where Leonidas's Spartans drew their final stand against the Persians, you would now stand on dry farmland with the sea barely visible on the horizon. Some of the later battles fought in the same pass, including the Roman defeat of Antiochus III in 191 BC and the ANZAC delaying action against the German invasion in 1941, took place on progressively wider versions of the same battlefield.

Modern Thermopylae is a small town in the Phthiotis regional unit, between the cities of Lamia and the village of Kamena Vourla. The A1 motorway, which connects Athens with Thessaloniki, runs straight through the pass along what was once the ancient shoreline. This stretch of highway is locally called the horseshoe of Maliakos, or sometimes the horseshoe of death. The road bends sharply, drivers travel fast, and accidents are common. The dark nickname is a grim modern echo of the older association with mortality. The springs still flow on the south side of the highway, near the foot of the hill where Simonides's epitaph for the dead Spartans is engraved into stone at the summit. The famous monument to Leonidas of Sparta, with its bronze statue raised in 1955, stands east of the highway, directly across the road from the engraved hill.

The 480 BC battle is the most famous, but the pass has been fought over again and again because the geography demanded it. Thermopylae was for centuries the only practical land route between northern and southern Greece, the only narrow place where an army moving south could be stopped or slowed. Athanasios Diakos and 48 of his Greek freedom fighters made a last stand near the bridge of Alamana in 1821, during the Greek War of Independence; he was captured and executed by the Ottomans. In World War II, ANZAC forces held the pass briefly in 1941 to allow the British expeditionary force to evacuate to Crete. German occupation documents later referred to the resistance sabotage of the Gorgopotamos viaduct in 1942 as the recent sabotage near Thermopylae. Every conflict that has needed to control central Greece has had to pass through here. Most have.

Visitors arriving at the modern site find themselves at a roadside monument complex with the Leonidas statue, an interpretive center, the engraved hill of Simonides, and the still-flowing hot springs. Across the highway, a long footpath winds along the foot of Kallidromo to the so-called Kolonos hill, where the last Spartans and Thespians made their stand. Excavators recovered hundreds of bronze and iron arrowheads on this hill, exactly as Herodotus described, evidence of the Persian archers who finally killed the surrounded Greeks. A signposted bilingual marker quotes Simonides: Go tell the Spartans, stranger passing by, that here, obedient to their laws, we lie. The springs still smell of sulfur. The Malian Gulf is visible in the distance. The scale of the original landscape requires imagination to recover, but the geography that made the place strategically critical is still there, written in limestone.