Kashshaptu

Description
Kashshaptu is the first terrestrial planet the rachni visited after achieving spaceflight. Although most of Kashshaptu is composed of silicate minerals, the rachni made an important discovery when they charted "the houling gulf," an enormous crater near the planet's equator. The depression was caused by the impacet of several dozen extrasolar asteroids containing large quantities of Element Zero. The rachni took samples of the new element back to Suen for further study.

Centuries later, when the rachni captured the Citadel expedition that opened the relay to their system, their research on eezo helped them reverse-engineer the ships' FTL drives. The rachni built faster-than-light vessels of their own and rapidly expanded beyond their own stars.