They were on the other side of our flat earth. The upside down part of Earth.

Unfortunately, when the Earth flipped over due to the reverse of the polarity, they simply fell off of the Earth.



Duh.

Another possible theory is that they were raped by yetio, which we all know, also have the ability to do the transdimensional thing. Over the ages, the new crossbred Tartarians were able to do the transdimensional thing as well and just said "screw you guys, we are outta here." and left. To another dimension.

Again, duh.

Any more stupid questions?