The CORE is a power production complex that produces all the power in The Underground. It was developed and constructed by Dr. W. D. Gaster in 1604. The process it uses to produce so much power basically turns it into a miniature sun. Gates are opened at certain times to allow light to pour out to light up The Underground.
The CORE, unbeknownst to anyone else, is just a façade for the many experiments Wing conducts within its walls. Many unauthorized experiments have happened there, including many with time-travel, and many that have to do with getting though the Barrier. The fact that the CORE is a geothermal plant is also just a façade.
The Process of Energy Generation (PEG)
In UTR, the process of which the CORE uses to generate power is explained below. For a more in-depth analysis, see Nuclear Fusion.
When the Ice gets to the CORE, it falls deep into it where it is converted to steam. This steam is then sent through a network of pipes that eventually hits a magical field that divides the Hydrogen and Oxygen in the water vapor into Hydrogen Gas and Oxygen. The Oxygen is then sent through an exit pipe. The Hydrogen is sent through another network of pipes that goes through another magical field that makes sure that ONLY Hydrogen gas enters into it. The Hydrogen Gas is then broken into just Hydrogen atoms just before they hit the fusion room. In the fusion room, the Hydrogen atoms are smashed together into Helium to make a Nuclear Fusion reaction. The energy produced from the Nuclear Fusion reaction is used to produce power for the rest of the Underground. The Helium produced from the reaction is then super-cooled with magic, and is used to cool major things like the CORE, and other industrial complexes like the server rooms that hold the Undernet.
The CORE during the Underearth Arc ended up being a staging ground, crawling with mercenaries.