Are Yajuj Majuj on earth?

I've heard that the nation of Yajuj and Majuj (Gog and Magog) is in the ratio 999:1 to humans, meaning there are 999 of them for every human. There are approximately 7.8 billion humans on the earth today. This would mean there are approximately 7792.2 billion of the Yajuj and Majuj, which is a number beyond human comprehension. It seems impossible for such a large group to remain hidden and undiscovered for so long. So are the Yajuj and Majuj really on earth or are they in a completely different place? If they are on earth, how is this possible?