The Core, It's very strange and mysterious. What causes it power? Who built it? Why is it there? Well, I noticed something when looking at the core and the clockworks... a strange blue glow. While most people associate lime green glows with radiation. Radioactive particles smash with other particles and may form a light blue glow in the air. There are two types of radiation that cause this. Cerenkov Radiation, and Spectral Radiation. Cerenkov radiation is seen when beta particles go through something dense, such as water. Well why would we be seeing this? the Core isn't in any water! It could be that Cradle's atmosphere is much more dense than what would be on Earth. The more likely thing would cause the blue glow is a phenomenon known as Spectral Emissions, which forms a blue glow in the air around highly radioactive objects. Spectral emissions are caused by ionization of oxygen and nitrogen in the air, similar to the effect of the Aurora Borealis which is formed in a similar way. This could form easily, even if Cradle's atmosphere is similar to a habitable planet planet such as Earth, which would make it easier for all the life that lives on cradle to breath without help from respirators (Which makes sense due to wolvers and other natural creatures that were brought here from wherever by whoever built cradle can seemly breath fine). Another idea to support that The Core is a Nuclear reactor is the steam that is seem everywhere and released in higher levels. This steam could be heated up water that is being exchanged for cooler water to cool the core. There are many obvious holes in my theory, but it's a start.
This was the only picture I can find on the wiki with a good look at the core. Feel free to post your own screenshots. (Blue glows in freeze levels don't count)
The Core, letting off what may be spectral emissions.
http://media.spiralknights.com/wiki-images/b/b0/Core_Overlook.png
Loading screen, blue glow from the depth
(Working on getting the picture)
All pictures belong to their respective owners.
Open a window, look at the sun. A giant, natural, nuclear reactor.
It doesn't need water (nor a cooling system) and it emits light.
Spectral emissions? Cerenkov radiation? Don't over analyze it.
It's a blue star with gravity dampeners and heat absorbers built around it. Sci-fi style/videogame logic.
We all know the core it's a sort of reactor (it emits energy, duh). The question would be how it came into existance.