Therum

Description
Therum is a distant but rich industrial world claimed by the human Systems Alliance. Its plentiful heavy metals have fueled the recent manufacturing boom on Earth. Core samples rich with the fossils of simple silicon-based organisms indicate Therum was more habitable in the past than it is at present. Perhaps this explains the many Prothean ruins dotting the surface, most of which have been looted by mining corporations.

Find Liara T'Soni
One of the key missions. Dr. Liara T'Soni, Matriarch Benezia's daughter and an expert on the Protheans, is somewhere in the Artemis Tau cluster. Commander Shepard goes looking for her to see what she knows about the Conduit, and to find out if she is in league with Saren Arterius.

When Shepard's team lands on Therum, Joker says he's picking up strange readings from an underground position a few clicks from the drop zone. They head there in the Mako, encountering several Geth Armatures on the way and a heavily armed gatehouse with several turrets. Shepard leads the team around the turrets and launches an attack from behind to clear the way. Finally they reach a narrow valley and have to continue on foot.

Reaching the entrance to an underground dig site, a Geth Hopper left on guard is their only warning. A dropship flies overhead, dropping several hoppers and Geth Shock Troopers, and another Armature. After a pitched battle Shepard's squad destroys the geth and heads underground.

They take elevators to the bottom of the dig site, encountering geth patrols and discovering that several barrier curtains have been set up, blocking off the tunnels. After one elevator breaks down, they find Liara T'Soni behind one of the fields, trapped in some sort of energy bubble, an ancient Prothean defence that she triggered by accident. She warns them to be careful: she set off the defences intending to protect herself from the geth. They are all over the dig site, and there is a krogan who seems to be leading them.

Shepard hacks a heavy mining laser to bore a hole through the walls, allowing the squad to reach Liara and turn the defences off. Liara asks why the geth are hunting her - the squad begins explaining about Saren's hunt for the Conduit, but a rumbling noise cuts the conversation short. The mining laser has destabilised the ruins and they are beginning to collapse. Shepard calls Joker for an evac and they take the elevator up to the surface.

Once they reach the top, the squad is confronted by a krogan battlemaster and his geth. He demands that Shepard hand Liara over. Shepard doesn't like this idea and the squad take him out. But by now the whole ruin is raining debris down on them. Shepard's team and Liara race through the dig site, dodging falling rocks and collapsing walls, reaching the surface just in time where the Normandy is waiting for them.

During the debrief, Liara is shocked by Joker's flippant attitude toward their near-death encounter. Under questioning, she explains that she knows very little about the Conduit - her area of expertise is the Prothean extinction, and while she knows the Conduit was somehow involved, she has no other information to offer.

Liara tells Shepard that according to her findings, the Protheans were not the first galactic civilization to suddenly vanish, but that there is a cycle of extinction that has continued for millions of years. Liara has dedicated her life to discovering the truth behind this cycle and the fate of the Protheans. She is fascinated by Shepard's vision, the description of the Prothean Beacon and the Reapers, but her ordeal has exhausted her and she needs to rest.

Afterwards, Shepard can speak to the Council, who aren't thrilled that a major Prothean ruin was destroyed. They also warn the commander to be careful of Liara T'Soni. She might have helped them but she is still Matriarch Benezia's daughter.

Alternative scenario
If this mission is left until last, Liara's reactions are very different. Down in the Prothean ruin, she refuses to believe Shepard and the team aren't a hallucination until they actually release her from the energy bubble, claiming that she's just conjuring up a protective figure like a Spectre to stop herself from going mad. She cringes in fear from the krogan battlemaster, begging Shepard not to let him take her.

During the debrief, as she hears about the evidence of the Reapers, the Cipher from Feros and Shepard's visions from the Prothean Beacons, Liara gets angry that all her decades of research have been wasted - in a short period of time the squad have stripped all her discoveries away. She begins to calm down when she realises that her expertise is the only way for Shepard to interpret the vision, and joins the squad to help in the upcoming battles.

If you complete the assignments on Feros and Noveria prior to rescuing Dr T'Soni, you will still receive the hallucination dialogue, however she will not react fearfully to the Krogan Battlemaster.

Tactics
See the Mass Effect Guide for a detailed walkthrough.

Trivia

 * In the demo footage for Mass Effect, there was a mission on a planet named Caleston that involved rescuing a group of miners who had been trapped underground by the geth attack. This was later merged into the mission on Therum. The approach to the underground entrance and the battle outside are the same, but in the demo Shepard wanted to know why the geth would drop an Armature just to wipe out a group of miners, and decided the site was worth checking out.
 * Therum was originally intended to be home to an entire Eldfell-Ashland Energy mining facility, complete with on-site workers and a seedy bar (as mentioned in the The Art of Mass Effect).
 * The krogan battlemaster's response to Shepard pointing out the ruins are collapsing - "Exhilarating, isn't it?" - is a quote from Star Trek III in a similar situation during the collapse of the planet Genesis.
 * When on Therum's surface, suffering from the planet's incredibly hot climate, Alenko jokes: "At least it's a dry heat." This is a reference to Aliens, where Hudson says the same thing about the alien nest under the atmosphere processor.
 * On the PC version of Mass Effect, large areas of land on the planet's surface may not render properly causing swaths of landscape to be a black hole, although the topography and ability to traverse these areas is retained. This error can be fixed by using repair feature of the configuration utility and deleting local shader cache files.
 * The capital Nova Yekaterinburg ('New Yekaterinburg') may be named after the Russian metropolis located near the Urals.