Inferno

"Inferno" is the nineteenth episode of the second season of Stargate: Atlantis.

Synopsis
On Taranis, the Atlantis expedition encounter an Ancient outpost currently used by the Taranians to protect themselves against the Wraith. However, the outpost's use of Geothermal energy awakens the planet's supervolcano, which is set to blow any minute.

Plot
Atlantis is contacted by a planet that claims to have Ancient technology and is in need of their help to fix it. When Lt. Colonel John Sheppard and his team arrive on the planet they find that there is actually an Ancient outpost which is protected by an Atlantis size shield, but that the inhabitants have apparently run into a problem operating it.

The problem, however, turns out to be much bigger than merely powering the shield. As it turns out, the outpost has Long range sensors just as Atlantis does, and after detecting the increased Wraith activity a year ago, the people in the outpost began running the shield continuously. The problem was, however, that the shield was powered by Geothermal energy and never designed to be used continuously.

As Dr. Rodney McKay discovers, what the people thought to be a simple geothermal pocket was in fact a Supervolcano, and what they thought was a large valley, turned out to be the caldera of this volcano. Because of this revelation, McKay determines that the extended use of the shield over the last year, and the resulting drain on the geothermal energy of this volcano, has critically accelerated the eruption which would normally not have occurred for up to 100,000 years.

McKay is now in a hurry: he not only has to fix the shield generator but also an Ancient warship that is docked in an underground bay, which is of the same class as the Aurora (see "Aurora"). After this discovery Dr. Elizabeth Weir offers to evacuate the populace to Atlantis, but Chancellor Lycus is skeptical. However, he agrees when the earthquakes become more frequent and the ground rips open.

While transferring the first wave of refugees, a lava pocket opens up directly under the Stargate, burying it under molten rock. Because of this, Atlantis contacts the Daedalus, which is just returning from Earth, and re-routes it to this planet. The Daedalus, however, cannot evacuate everyone at once, and must make four trips. McKay feels this will take too long and while the Daedalus evacuates the most important people he decides to try to repair the engines of the Ancient ship and simply fly off of the planet. Unfortunately, some of the natives grow impatient, and take their chances seeking refuge in the wilderness because they don't believe that the eruption of the volcano will destroy the entire continent. Teyla Emmagan and Ronon Dex try to evacuate the last of the natives, but the tunnel leading to the Ancient ship is blocked. Fortunately, they are beamed up to the Daedalus before the poisonous gas released by the volcanic activity can kill them.

Meanwhile in Atlantis the expedition team learns from Dr. Radek Zelenka that a Wraith Hive Ship is on its way towards the city, increasing the pressure on McKay to successfully repair the Ancient ship. However, they know that even with their resources they might not be able to defeat the enemy ship.

In the end, however, McKay is unable to make the repairs in time and has found that a river of lava has overrun the bay doors, making it impossible to fly out anyway. Instead he re-routes power to the hyperdrive engines and activates the shield. But, due to the high temperatures and pressures of the lava that will soon envelop them, the shield will only last for four seconds. These four seconds will give McKay enough time so that the explosion of the volcano will thrust the ship, which Sheppard named the Orion, high enough to engage the hyperdrive for a microsecond, propelling them into orbit, and saving everyone on board. Upon returning to Atlantis, McKay and a team of scientists begin working on the Orion in hopes of fully repairing it.