It is the first example of its kind to come to light in Britain, and reveals fascinating new details about pagan religious thought in the late Roman period.
It suggests that the long-
established classical cult of Achilles may eventually have been incorporated into Christianity's major late-Roman competitor - the Orphic religion, whose pagan saviour-
figure, Orpheus, was allowed by the gods to cross over into the underworld and then return to the land of the living.
The Humberside mosaic was discovered in 1795 by labourers preparing a kitchen garden near the village of Horkstow, 30 miles north of Lincoln.
It was 50ft long and 25ft wide and, when it was constructed in the mid-fourth century AD, it would have functioned as the floor of a large unheated hall, in part of a substantial villa-type complex, overlooking a tributary of the river Humber.
The mosaic, which is divided into three parts, was then examined and drawn by two mutually hostile 18th-century antiquarians, William Fowler and Samuel Lysons.
Lysons concluded correctly that the western portion of the pavement was dedicated to the Greek mythological figure Orpheus. He thought the central and eastern parts of the mosaic portrayed bacchanalian scenes and an ordinary chariot race respectively.
Now, almost 200 years later, new research by a Bristol Art Library classicist, Anthony Beeson, has revealed that Lysons was almost certainly wrong, and the central and western portions of of the Horkstow mosaic represent scenes from the life of the Trojan War hero Achilles.
The scenes are those described in Homer's Iliad or in later Roman works of poetry. The extant remains appear to portray four key stories in Achilles' life.
The first legend featured is the manner in which Achilles' mother sought to make her son immortal. The partially preserved mosaic image probably showed Achilles' mother Thetis dipping her newly born child into the underworld's river - the Styx - to give him eternal life on this earth.
Tragically she forgot to wet the heel by which she held him - and years later, at the siege of Troy, it was an arrow in the now famous 'Achilles' heel' that laid low the greatest of Greek heroes.
The second legend featured in the mosaic is probably that of the great romance between Achilles and a Greek princess, Deidameia, in whose father's court Achilles had been hidden by Thetis. Once again Thetis was trying to defy fate - this time by turning her son into a draft-dodger, by hiding him so he would not have to join the Greek army and fight at Troy.
However, Achilles made Deidameia pregnant and blew his cover. Appropriately, the Greek goddess of love, Aphrodite, is portrayed (in a seashell) adjacent to this scene. In deliberate contrast is the third scene, which describes a story of utterly tragic love - perhaps the world's briefest and saddest romance.
It portrays Achilles killing Penthesilea, the queen of the Amazons, who had led her female warriors to the aid of Troy. Tragically (but to no avail), they fell in love at the very moment that Achilles killed her with his sword.
Two other mosaic images have not survived, but Mr Beeson suspects that the now- obliterated scene opposite the 'dipping in the Styx' legend was probably an image of the death of Achilles outside the walls of Troy.
In the central roundel of the mosaic, he believes there would have been some other famous legend of Achilles - perhaps the story in which the goddess Minerva grabs Achilles by the hair to stop him becoming angry with the Greek king Agamemnon.
The fourth surviving part of the mosaic, now also identified as probably featuring Achilles, is the western portion - that portraying the chariot race. This, like the Penthesilea story, also appears to relate to a tale of tragic love.
Achilles' greatest male companion, Patroclus - whom he dearly loved - was killed when Achilles refused to fight the Trojans and Patroclus went and fought in his place.
The mosaic appears to portray the chariot race organised by Achilles to mark the funeral of Patroclus. Homer describes in the Iliad various details which are faithfully reproduced in the mosaic.
These include an incident in which a charioteer is thrown from his chariot, and another in which Achilles attempts to present the same charioteer with a prize in the form of a horse.
Classical paganism experienced a substantial revival in the mid-fourth century AD. Although Christianity had been the official religion of the Roman empire for several decades, paganism was briefly re- installed as the official religion around 360 by the emperor Julian.
The philosophy behind late-Roman paganism was what is now called Neoplatonism. It was obsessed with exploring different levels of existence and reality. They looked at it from the very complex to an ultimate state of 'being' so fundamental and sublime that it was quite literally 'beyond being'.
Neoplatonism was also obsessed with circularity - the concept of going out in order to return. Many of these ideas fed into early Christianity, and lived on to influence medieval and modern philosophical and psychological thought.
Perhaps the most signicant mythological figure featured at the religous end of neoplatonic thinking was Orpheus, who died at the hands of jealous women after journeying to the underworld to rescue his beloved wife.
His now-famous glance back at his wife consigned her to everlasting death, while he lived on in sadness, destined in the end to become one of the Judges of the Underworld.
His character combined and reconciled the otherwise contrasting characters of the other stars of the Orphic - Bacchus and Apollo. He became almost an intermediary between this world and the next - a sort of pagan saviour, a competitor of Christ in the fourth century.
Late Roman neoplatonic thought and the Orphic religion are still, to a large extent, a mystery. Archaeological discoveries and a tiny corpus of writings (mainly by Julian and Macrobius) shed only limited light on the last great lost religion of the pagan classical world.
The reinterpretation of the Horkstow mosaic (probably part of an Orphic chapel or sanctuary) add just one more piece to the still-incomplete jigsaw of our knowledge of Christianity's last great rival.