Goa'uld who had occupied the planet Langara (Kelowna) some 3,000 years ago, leaving behind a data crystal that detailed plans of his naquadria research project. Thanos had managed to create a small amount of naquadria, a powerful variant of naquadah, but was not careful enough in experimenting with the volatile substance. The resulting explosion released subatomic particles into the crust of Langara, catalyzing two massive veins deep into the core.
While his experiments created the substance that would make Langara a prime target for the Goa'uld and threatened to one day destroy the entire planet, Thanos' creation of naquadria also indirectly led to Earth's development of ships capable of travelling through hyperspace, including the F-302 and Prometheus.
Thanos's final fate is unknown, but it is likely that he was killed in the explosion that resulted from his research.
Fallout - Jonas Quinn returns to Earth, updating the S.G.C. on what he learned about Thanos, the Goa'uld that once used Kelowna (now Langara) as a test base for unstable naquadria.