They originate from the century-long Explorator missions conducted by Thule into the very edges of the Imperium territory. During that time, he had many students in the form of Adepts and Magos which served underneath him. These students later departed from explorator fleet and stayed on locations of archeotech remains and vital resources. Many of them could become forge worlds if Adeptus Mechanicus would pour enough resources for their development.
As Explorator fleet of the famous magos disappears to Rift of Hecaton, over century ago, many sites claimed and data gathered by his disciples are watched with hungry eyes among influential at Koronus Expanse and Calixis Sector.