What happened to Tom Bombadil when the last ship left for the Undying Land?

Question

When the wizards and elves leave Middle Earth the only great power that is left will be Bombadil. Towards the end of The Return of the King, when Gandalf leaves the hobbits, he mentions that he wants to have a long talk with Bombadil.

‘But if you would know, I am turning aside soon. I am going to have a long talk with Bombadil: such a talk as I have not had in all my time. He is a moss-gatherer, and I have been a stone doomed to rolling. But my rolling days are ending, and now we shall have much to say to one another.’

So what became of him?

Answer 1

Letter 144:

And even in a mythical Age there must be some enigmas, as there always are. Tom Bombadil is one (intentionally).

The end of Lord of the Rings is not a traditional wrapping-up of loose-ends and Bombadil is one that is (perhaps intentionally) left hanging. There's no mention of a subsequent history for him in either the Letters or the HoME volumes; that doesn't mean that such a subsequent history doesn't exist, of course, just that Bombadil's part in the story was done.

The nearest hint we get is in Gandalf's response to Frodo shortly after the passage you've quoted:

'As well as ever, you may be sure,' said Gandalf. 'Quite untroubled and I should guess, not much interested in anything that we have done or seen...'

Bombadil's part in the world is not to be involved in the Great Events; he doesn't get involved and they don't interest him. I'd even dispute that he's a "great power": he's very much a stay-at-home who doesn't get involved. This is also well-summarised by the discussion of him at the Council of Elrond:

'He is a strange creature, but maybe I should have summoned him to our Council.'
'He would not have come,' said Gandalf.

'Could we not still send messages to him and obtain his help?' asked Erestor. 'It seems that he has a power even over the Ring.'

'No, I should not put it so,' said Gandalf. 'Say rather that the Ring has no power over him. He is his own master. But he cannot alter the Ring itself, nor break its power over others. And now he is withdrawn into a little land, within bounds that he has set, though none can see them, waiting perhaps for a change of days, and he will not step beyond them.'

'But within those bounds nothing seems to dismay him,' said Erestor. 'Would he not take the Ring and keep it there, for ever harmless?'

'No,' said Gandalf, 'not willingly. He might do so, if all the free folk of the world begged him, but he would not understand the need. And if he were given the Ring, he would soon forget it, or most likely throw it away. Such things have no hold on his mind. He would be a most unsafe guardian; and that alone is answer enough.'

So Bombadil is definitely someone who doesn't get involved, and any subsequent part he may have to play will most probably be along the same isolationist policy.

Answer 2

There has been non-canonical writing in the Iron Crown Enterprises Middle Earth Roleplaying Game that cast Tom as a Maia, one of the Ainu who came into the world in the beginning, and that he was bound to the Old Forest.

Generally non-canon because it was never published in any of the official works by J.R.R. or edited by his son Christopher that I am aware of. But almost all of the game material was licensed by Tolkien Enterprises and was generally reviewed and approved for release by Christopher from 2nd Edition on.

I say from 2nd Edition because Iron Crown had taken some creative license with their 1st Edition that conflicted with how J.R.R. portrayed Middle Earth. Which was understandable to a point in order to fit their game mechanic, and provide greater depth and enjoyment to the game. But their 2nd Edition was held to higher standards. An example of text included in 2nd Edition, specifically from Southern Gondor: The People, that shows this:

THE SOURCES This is an authorized secondary work. It is specifically based on The Hobbit and The Lord of the Rings, and it has been developed so that no conflict exists with any of the other primary publications. Of course, always remember that the ultimate sources of information are the works of J.R.R. Tolkien. Posthumous publications edited by his son Christopher shed additional light on the world of Middle-earth. Southern Gondor: The People is based on extensive research. We uphold the high standards associated with the legacy of John Ronald Reuel Tolkien. By blending material from primary and secondary sources with rational linguistic, cultural, and geological data, we insure that any interpretive material fits into Tolkien's defined patterns and schemes. Keep in mind, however, that this is by no means the "sole official view." Since we derive the material in Southern Gondor: The People from authorized sources, we provide citations to pertinent sections in The Hobbit and The Lord of the Rings and other major works by J.R.R. Tolkien. Where we have extrapolated information, we either omit citations or we refer to publications in ICE's Middle-earth adventure game series.

That isn't to say that a lot of their 1st Edition material wasn't republished in their 2nd Edition.

Answer 3

The elves, the men, the hobits, etc. are all creatures living upon middle earth and whose existence is essentially independent of the existence of middle earth. Bombadil is a part of middle earth, just as are the mountains and rivers. When middle earth ceases to exist, so will Bombadil because he is a part of it.