Cover arc story.
Funnily enough it doesn't bother me at all. I like a bit of unexplained mystery in my story. If Oda explains that, I'd also want him to explain how dinosaurs lived on an island still, how Sea Kings came to be, how did someone build Impel Down to reach all the way to the bottom of the ocean and keep it in the calm belt.
There are just some things that will never be explained. Still too early to truly say whether or not Oda will never explain that thing, especially since we've gotten no cover arcs concerning the Thriller Bark arc yet and the SH's are still in the first half of the grand line. But that said, I believe the probability that it will remain a mystery is high.
Kind of Oda's tribute to the Bermuda Triangle's continued mystery.