- The organic core is the center of Cybertron in the Beast Machines portion of the Generation One continuity family.
For countless eons, Cybertron has existed as a world of metal. It was not always so. Long before Primus imbued life into the Thirteen original Transformers (or the Quintessons arrived, your pick), Cybertron was a world of the organic, with animal lifeforms that roamed its surface. That is gone now, but the past remains in the organic core of Cybertron.
When the Maximals were trying to set up an organic orchard in the fossil cave, they eventually struck paydirt when they reached part of the organic core. Upon hitting the core, organic goo seeped out, causing trees to emerge.
Later, after Megatron had absorbed the sparks of the Transformers, he attempted to use the Key to Vector Sigma to convert the organic core into technomatter. However, Optimus Primal caused both Megatron and himself to fall into the organic core. Hitting the core caused them both to die, but also allowed the Oracle to initiate the Great Transformation, causing Cybertron to become a Technorganic world.
Japanese cartoon continuity
The organic core of Cybertron was due to the Oracle who had fled Primacron and brought life to a barren planet, making it lush and green. The Oracle was later enslaved by the Quintessons, who transformed the planet into a mechanical world and the Oracle itself into Vector Sigma.