Hellespont (Turkish , Greek ; i.e. "Sea of Helle", variously named in classical literature Hellespontium Pelagus, Rectum Hellesponticum, and Fretum Hellesponticum) was the ancient name of the narrow strait, now known by the modern European term 'the Dardanelles'. It was so called from Helle, the daughter of Athamas, who was drowned here in the mythology of the Golden Fleece. The Hellespont is one of the Turkish Straits, along with the Bosporus, that separate Anatolia from the continent of Europe.
Herodotus tells us that c. 482 BC the king Xerxes I of Persia (the son of Darius) had two bridges built across the width of the Hellespont at Abydos in order that his huge army, ostensibly made of 5 million men (most historians put the actual number of this army at closer to 250,000 men, though a second school of thought lends the accounts of Herodotus more credence, bringing the number closer to 400,000), could cross from Persia into Greece. This crossing was named by Aeschylus in his tragedy The Persians as the cause of divine intervention against Xerxes.

The Hellespont was also the body of water which Leander would cross in order to tryst with his beloved, the priestess Hero. Lord Byron famously swam the Hellespont as a feat of his athletic prowess.

Xerxes' Crossing

According to Herodotus (vv.34), both bridges were destroyed by a storm and Xerxes had those responsible for building the bridges beheaded and the strait itself whipped. The Histories of Herodotus vii.33-37 and vii.54-58 give details of Xerxes' building and crossing of the bridges. Xerxes is then said to have thrown fetters into the strait, given it three hundred lashes and branded it with red-hot irons as the soldiers shouted at the water.

Herodotus commented that this was a "highly presumptuous way to address the Hellespont" but in no way atypical of Xerxes. (vii.35)
Harpalus the engineer eventually helped the invading armies to cross by lashing the ships together with their bows facing the current and two additional anchors.