A subterranean being matrix is one of the synthesis matrices that Titans use when seeding a world. Each synthesis matrix is used to achieve the Titans' goals. Typically, the Titans apparently use a "biomass construction foundation" in a subterranean being matrix (likely meaning flesh).
For Azeroth, a subterranean being matrix was to be modified to create a prototype with increased durability with an affinity for deep region shaping. Their design intent was an attempt at improving the survivability paradigm of the standard subterranean being matrix. Through study and research of Azeroth's planetary crust, the Titans discovered that Azeroth has a inherently resilient composition. They then theorized that if they used a cross-section of the crust as the foundation, a subterranean being could be made that would epitomize the resiliency of the planet's essence. Thus, they used a blend of Azeroth's various stone core compounds to synthesize a lifeform that satisfied the design parameters they had set out. The new race and the product of this modification was appropriately named "Earthen".
Well into the Earthen's primary service cycle for their Creators, unforeseen anomalies were discovered which affected their synthesis matrix's destabilization due to high-stress environments. The Titans began work on post-synthesis modification, rather than recall and re-synthesis. (Since the Earthen's only downside was the destabilization of their matrix, the Titans prefered to perfect their matrix rather than start over from scratch.) The first series of experiments resulted in troggs. The Series Two experiments however, resulted in dwarves, a reversion to a default biomass composition as seen in the standard subterranean matrix.
Further experiments conducted in Ulduar revealed that the "high-stress environments" were the Old Gods purposely affecting matrix destabilization, which the Titans termed as the "Curse of Flesh". Once the Old Gods were neutralized and contained within Azeroth, the Forge of Wills and other systems were instituted to create new Earthen. As the Titans now knew the cause of the Curse of Flesh, the new Earthen were able to be created with immunity to it. Even as the old Earthen would become troggs and dwarves, the new Earthen remained unaffected and still continue to exist in modern Azeroth.