The story is older than most of the books that carry it. A ship sights land where no land should be. The crew goes ashore, hammers in mooring stakes, lights a cooking fire on the sand. The warmth reaches what is under the sand, and the island sinks with everyone still on it.
In the Greek Physiologus, compiled around the second century, the animal is called aspidochelone and the tale is told as a warning: the sea beast is the devil, the island is worldly comfort, the stakes are false faith. The Arab tradition calls the same creature zaratan and drops the moral. Sindbad the Sailor lands on it in his first voyage. Irish monks meet it in the Voyage of Saint Brendan, where it turns out to be a fish named Jasconius, and they return to it every Easter for seven years.
Why it travelled so far
Because it is the shape of a real fear, told in the shape of a real thing. Whales do surface for long periods, barnacles do grow on their backs, and a becalmed crew in an open boat will believe in land when they see it. The tale needs no monster to start — only a whale, a horizon, and men who very much want to stand on something solid.