The inner earth is a highly advanced civilization. They are at a different dimension and they don't desire to inter mingle with earth humans because earth humans are too violent. They have a physical body but less dense than the earth human body due to their high spiritual evolution.
The access entry point to inner earth is at the polar. The distance between earth surface and the inner earth can not be measured by our 3rd dimensional method. Because the distance involves dimensional difference. There is parallel earth which exists in another dimension is vastly different from the 3rd dimension earth surface. Time and space are understood differently based on what dimension you are in.