Its very easy to explain. The humans in inner earth are the human race of evolution nr. 8 and you all on the surface are the human race of evolution nr. 9 so they were the last race that got into inner earth 16 billion years ago. They had enough time to create a second race and other beings from other solar systems stated also bases in the inner earth.

Than in the last century the human race of this evolution started also underground bases in inner earth, beside the human race of the last time-line. These you would call nordics, but they are not aliens. They live since more than 8 billion years in inner earth.

The experimental animals get just created to study genetic science and how to modulate life on genetic scales. Civilisations that are billions of years ahead of the intelligence of the so called modern human from today all do genetic engineering and experiments with life. Just some study life "without touching it" but mostly all highly advanced civilisations reach a point were they play god because they realize that they ARE god(s). The modern humans is not ready for this experience, to feel like a god, because a civilisation from the orion-group which belongs to the reptilian empire burned the factors away in the human dna that are necessary to access higher powers and higher awareness.

Just the negative orientated humans and creatures destroy life but they do it just because their genes are degenerated so they are on their best way to get a race of androids because they will get without the higher sources just a empty body with a erased or extremly degenerated mind that doesn´t work correct or at all. Other beings that are inorganic will use such beings to visit the physical realitys of this and other planets.

Mars is full with underground bases. Venus, pluto and neptun also got underground bases. Moon just got created to remote control the activity of life on earth bilion years ago.