The origin of the Celtic Elysium belief may be found in universal myths of a golden age long ago in some distant Elysian region, where men had lived with the gods.  Into that region brave mortals might still penetrate, though it was lost to mankind as a whole.  In some mythologies this Elysium is the land whither men go after death.  Possibly the Celtic myth of man’s early intercourse with the gods in a lost region took two forms.  In one it was a joyful subterranean region whither the Celt hoped to go after death.  In the other it was not recoverable, nor was it the land of the dead, but favoured mortals might reach it in life.  The Celtic Elysium belief, as known through the tales just cited, is always of this second kind.  We surmise, however, that the land of the dead was a joyous underworld ruled over by a god of fertility and of the dead, and from that region men had originally come forth.  The later association of gods with the sid was a continuation of this belief, but now the sid are certainly not a land of the dead, but Elysium pure and simple.  There must therefore have been at an early period a tendency to distinguish between the happy region of the dead, and the distant Elysium, if the two were ever really connected.  The subject is obscure, but it is not impossible that another origin of the Elysium idea may be found in the phenomenon of the setting sun:  it suggested to the continental Celts that far off there was a divine land where the sun-god rested.  When the Celts reached the coast this divine western land would necessarily be located in a far-off island, seen perhaps on the horizon.  Hence it would also be regarded as connected with the sea-god, Manannan, or by whatsoever name he was called.  The distant Elysium, whether on land or across the sea, was conceived in identical terms, and hence also whenever the hollow hills or sid were regarded as an abode of the gods, they also were described just as Elysium was.

The idea of a world under the waters is common to many mythologies, and, generally speaking, it originated in the animistic belief that every part of nature has its indwelling spirits.  Hence the spirits or gods of the waters were thought of as dwelling below the waters.  Tales of supernatural beings appearing out of the waters, the custom of throwing offerings therein, the belief that human beings were carried below the surface or could live in the region beneath the waves, are all connected with this animistic idea.  Among the Celts this water-world assumed many aspects of Elysium, and it has names in common with it, e.g. it is called Mag Mell.  Hence in many popular tales it is hardly differentiated from the island Elysium; oversea and under-waves are often synonymous.  Hence, too, the belief that such water-worlds as I-Bresail, or Welsh fairy-lands, or sunken cities off the Breton coast, rise periodically to the surface, and would remain there permanently, like an island Elysium, if some mortal would fulfil certain conditions.[1256]