Some would be surprised by my choice that first comes to mind.... the ten lost tribes of Israel.
I first came upon the topic while doing research many (many!) years ago doing a school project. While looking into the that area of the research is also when I started to go back and read a lot of the early works by Isaac Asimov. What's the connection? While many know Asimov strictly as a sci-fi writer he was also a chemist professor who is as well known in academic circles for his non-fiction works as he is in the sci-fi circles for his works of fiction. Among his works is research into the lost tribes.
So, yeah, there you have it.... if time & money were of no concern then I would pursue researching & writing about an ancient mystery.