Taranis was a planet in the Pegasus galaxy. It was a former colony of the Lanteans, as evidenced by the outpost left behind with a dialing device like the one used in Atlantis, and possessing a shield being powered by geothermal energy.
During the War with the Wraith, Taranis served as a drydock for the Ancient fleet, with one ship, the Hippaforalkus, being left behind after the Lanteans fled back to Earth. Eventually, Taranis was repopulated by humans, who began to study the Lantean technology left behind, and used the shield to protect themselves from the Wraith. However, the shield's employment of geothermal energy caused the dormant super volcano the base was in to become active. The Atlantis expedition came to Taranis, and helped most of the people escape, but the Stargate was destroyed, and a massive dust cloud emerged from the volcano, rendering the planet uninhabitable for several centuries. (SGA: "Inferno")