One of the most famous elements of the tales of Arthur is the tradition that he did not really die but is merely sleeping in a cave, surrounded by his knights and waiting to return to save Britain when it needs him the most (not all versions of the tradition contain that final part). It is also one of the most fanciful elements. But, much like the arrangement of the Knights of the Round Table, this seemingly fanciful tradition may actually have a firm basis in reality (see this post for the historical evidence for the Knights of the Round Table).
This theory did not originate with me (I found it from Arthurian researchers Alan Wilson and Baram Blackett) but the intricacies of the theory and the reasoning behind it that I will present here are my own.
The theory is that this tradition about Arthur sleeping in a cave originated with the fact that Arthur was actually secretly buried in a cave. The evidence that this secret burial occurred is found in the ninth century Historia Brittonum and the 12th century Life of Saint Illtyd. For a brief overview of the theory, see my Facebook post here. For a more detailed analysis, read on.
According to the accounts in the above mentioned two sources, Illtud was living as a hermit in a cave for a number of years. One day, two men came in a boat and brought him the body of a 'most holy man' whose identity was to be kept secret from others. They then buried the man. Later, a church was built over the body (it was evidently removed from the cave and reburied). So, what are the reasons for believing that the corpse in this story was actually that of Arthur's?
Firstly, consider the fact that the most common tellings of Arthur's demise (including the earliest, in Historia Regum Britanniae) recount how he was taken away in a boat to the mysterious 'Avalon' after being mortally wounded. Whatever Avalon was or whatever happened there, it would tie in well if this record about Illtyd was essentially the other end of the journey.
Secondly, the description of the body as being that of a 'most holy man' is a good match for Arthur. The earliest records concerning him reveal that, as well as being a king, he also served a religious function among his people. He is portrayed as upholding the religion of his kingdom in Historia Brittonum, where he carries 'the image of Saint Mary on his shoulders' during one of the battles - and in the Welsh Annals, he is said to have carried 'the Cross of Jesus Christ on his shoulders' during another one of the battles. In fact, there is even a case to be made that Arthur was viewed as the 'Pontifex Maximus' just like the earlier Roman Emperors. In any case, the two Arthurian records show that was did have an important religious role in his kingdom and would thus most definitely have been considered a 'most holy man'.
Thirdly, the timing is important. This account is placed very near the end of Illtyd's Life, and the evidence indicates that he died somewhere around the year 580. In close correspondence with this, the majority of the evidence concerning Arthur shows that he did not die until the 570s (see this post and this one). Thus, the event concerning Illtyd and the cave is a perfect chronological match for Arthur's death.
Finally, in some other legends of Arthur, he is said to have been buried underneath a 'newly-built chapel'. This is a nice match with the detail in the account about Illtyd that a church was later built over the body of the holy man.
So if this account about Illtyd and the secret burial really is actually about Arthur, then it would explain many things. It would offer an explanation for where Arthur's body ended up after being taken away in a boat to 'the Isle of Avalon'. It would explain the stories of Arthur in a cave and why there is a fanciful, mysterious element to such stories (for the cave burial was kept secret, remember). And it would explain how, at the same time, there are tales of Arthur being buried under a newly built chapel.
