Great idea! I found the unknown megacity on my third guess, and quickly made it to the farthest being the only one left. Chengdu stayed in the back of my head, but I kept telling myself that it only had around 8 million population. I was wrong.
I tried megacities in China first. When I got Chengdu but not the others I started circulating to other countries in the approximate distance and finally got to the mystery city and the other closest cities.