A strange thing to say, and i will pull out the contentious parts.
Skin. "they want a piece of me, my blood, my skin" says a pale blonde woman in the Intruders film.
The underground races are said to have dark blue or brown skin. Trolls, and in Russian says Whitley this is 'Cobals'.
Miners, who like metal ore to make weapons and tools and they also use mercury to make mirrors. "Mirror Mirror on the wall, who is the fairest of them all?". This was a reference to the communication between the realms through "reflective" devices or surfaces.
A thought occured to me that maybe the underground races would be pale. pale travellers from the stars (the red beared Quetzecoatl) who would be adept at building, since they would require shielding fromthe harsh sunlight.
The Sphinx I was thinking last night. A woman and a Cat. A living stone ship which descended to earth. A race of people coming from a Queen and her Feline consort who had the power to levitate stones through sound.
This would explain why the later descendants of the pale skinned would be able to travel the world later on and make such an influence in the countries of the dark skinned, by which i mean anthropologists and biologists and acheaologists. Who not only travel and bring knowledge back, but also do things in the country of others which the residents did not think of or did not try. Anglo Saxon adventurers. Vikings. Empires. Was it only height, no since Nigerians are very tall. Surely there have been great people of all races. But not many if any Nigerians for example coming over to England and telling us about our past and our culture and our history. Not until now perhaps.
The dark skinned are more adept at surviving with what they have. Using Aborigines as an example, they are the oldest race of original humans so they say, we can see that they have survived for hundreds of centuries without houses, without cars, without structures of any kind except temoral temples and natural rock formations. They have been happy and provided for on the surface of the land. Due perhaps due to dermal burning, or an angsty temperament which brought them enemies or predators or something, other races have needed to shield and defend with walls.
Atomic or nuclear devastation has occurred before, causing mutations and polluting the earth to necessitate an escape route, possibly through the natural interface of rock caverns, to the source of the water spring.
Mercury is a liquid metal. Craftspeople skilled in the use and nature of metal and concentrating upon it for many years, may well have found a way to blend. think of the composits of bread and how they are 'kneaded' into a new consistency.
Crystal and metals found undergound, like the flowers and fruits of the surface.