The mastodons are a huge race of prehistoric beasts, which resemble elephants. As they roam through the primeval forest inside the earth, they are an awe-inspiring sight, but their keeper astounds the three travellers even more. This is because he appears to be a human being - a form of prehistoric man. He is a vastly imposing sight, 'over twelve feet tall', with an animal-like mane; but he seems to be human, however primitive. Axel and his companions have already seen fossilised remains of what appear to be humans, alongside those of animals; now they come across a living specimen and are simply overwhelmed. 

The three travellers encounter many amazing sights and events during their fabulous journey inside the earth, but it is the sight of this keeper which stupefies Axel more than anything else. Later, when he is able to think it over, he is simply unwilling to face the idea that there might be a race of humans living deep in the earth:

The idea that a man, a living man, and with him a whole generation, should be buried down there in the bowels of the earth is unacceptable. (chapter 37)

Axel, then, refuses to entertain the notion of an entire community of humans living in such isolated and primitive conditions, so completely cut off from the their counterparts on the surface of the globe. He takes refuge, instead, in the suggestion that it might have been some sort of human-like monkey. The thing that unsettles him most about this strange new world is the thought that he might have run into his own kind.