I like historical mysteries, e.g., something that we know has to have happened in some way, but about which we know very little, or even nothing. As I've been poking around to find some unrelated stuff out, I started getting distracted by the origin of the Malagasy language. Malagasy is the native language of Madagascar, and surprisingly, it isn't really related to any African languages at all except in regards to very late borrowings from Swahili. I've always been a fan of linguistics, and as I was checking out some stuff on the Austronesian language family, I was reminded that Malagasy is an Austronesian language. The Austronesian languages are believed to have all started on Formosa (Taiwan) long before there was any Han Chinese influence there, and spread largely south and east from there; Austronesian languages are the native languages of the Philippines, Malaysia, Indonesia, Melanesia, Micronesia and Polynesia. And then, in a geographical suprise, also Madagascar. And I think that's how I stumbled onto being reminded of this; four of the top ten islands (in size) are islands with these languages; three of them being part of Indonesia, and then Madgascar (four others are up in the Arctic; either Greenland or the neighboring islands of northern Canada.)
In fact, the Malagasy language can more specifically be related to Barito languages, which otherwise are found on the southeast of the island of Borneo. How some river-tribe guys from the opposite side of Borneo could end up settling Madagascar, contributing to its ethnic mix and specifically imposing their language on the entire island, even when other groups of people eventually made their way there too, is a historical or linguistic mystery. Which means that I therefore find it fascinating.
I think little things like this being part of fantasy campaign settings is something that most people don't think about. Why aren't there weird little historical mysteries that exist... just to be interesting?