[Lalwendë]

There is a pronounced theme of the 'underworld' running through Tolkien's work. Valinor is unique amongst 'retreats' in Arda, other races and even some of the Elves seem to favour retreat to the underground places.

Hobbits live in 'hollow hills', so it is not a difficult thing to imagine that as the years pass by they seek the underground places as suitable hideaways from the Big Folk. Dwarves too seek out caves, and with the retreat of the Balrog, maybe Moria became inhabited again. Elves historically had a liking for underground realms, and the Wood Elves of Mirkwood still lived that way. Even Men, in the case of the dead men of Dunharrow, find a suitable retreat in the underworld.

There is also a significant theme running through Tolkien's work where entrance to the Underworld leads to actual or symbolic rebirth, e.g. Gandalf in Moria and Frodo in the Barrow. It is a place at once of both safety and peril. This is very true to the ideas seen in myths and folklore.

Tolkien's conceit that his work is a translation of an ancient history of our own world mirrors real world myth and legend that Faerie can be found in the underworld. I suppose it is a case of trying to find the right door to enter that world.

It is interesting that he also used the idea of a mythical Western land far out to sea in the form of Valinor. That is a very Celtic idea, having both the Western, lost land and the land within the land.