My island is on Earth. It is my intention that it was a part of a larger landmass at some point during ancient history, so not as early as Pangaea. That’s important because the island’s inhabitants have passed down their origin story to the present day descendants, which tells that they lived on the land before it became an island, that they survived its ‘drift’ to sea, and have remained there ever since.
The island and its inhabitants went undiscovered by civilized society until the early 1900’s, when a wealthy American man stumbled upon it while circumnavigating the globe in his fancy yacht, or something like that... I’m adding this because I figure the fact that it went undiscovered for so long might influence answers regarding how the island drifted in the first place, where it might’ve come from, and where it might’ve gotten to. Also, for the same reasons, I’ll add that the island’s inhabitants are tribal, but because of the size of the island, or possibly just their culture, they live together in one tribe. As for any concerns about population that might pop up, the islanders sacrifice two tribe members 1-2 times a year for religious purposes, so let’s just assume that that takes care of that.
The island is tropical and it has one large river that weaves its way through the island. This river can be connected to the ocean or not; the important aspect of it is that the river was there before the drift, though I’m willing to change that if it’s impossible. The tunnels, on the other hand, need to have survived the drift somehow, because they contain preserved bodies of sacrificial victims dating back to the islanders’ original ancestors. I haven’t even considered composition of the tunnels, so if they have to be carved from some rare type of rock or if the islanders had to build them in some way for them to survive the drift, that’s fine.
So, my question(s) are: Is my island’s existence plausible? How could its inhabitants and their tunnels have survived the drift? Where could it have come from? How could its drift have occurred so that the aspects I’ve presented are possible? Where might it have ended up?
Note: (Just in case any questions arise on why I don’t just handwave this) I intend for a member of the tribe to explain their origin in detail at the beginning of the story. It’s an aspect of their culture that their history is so well preserved; despite the great length of time that they’ve lived on the island, they know what happened and have maintained their culture as near to its beginnings as possible.
In addition, if any of this is impossible without magic, that’s fine; I’ll accept an answer saying just that if it seems to be the best option. There is ‘magic’ of a sort in the story and I could use it to handwave this, I’d just prefer to be able to explain it realistically, so that the islander’s tale of their origins would sound plausible to a modern day person.