Subterranean Life is one of the quests you have to complete during the second Witcher 2 Assassins of Kings Enhanced Edition act, in order to save Saskia.
This specific quest, begins after The War Council main-quest, if you follow Iorveth.
By completing Subterranean Life you will be able to find one of the items required by Philippa to prepare a cure for Saskia.
The item is known as the dwarven immortelle.
“To call Philippa Eilhart a healer is like calling a drowner an advertiser of swimming. Nonetheless Philippa was a sorceress, a powerful one at that. Though healing was not her specialty, she could help the poisoned Saskia.
To create a cure, she needed special ingredients, a dwarven immortelle among them. Geralt started the search with asking one of his dwarven friends staying in Vergen. Zoltan, for he was said friend, tarried in the local inn.”
After you leave Saskias house, head directly towards the inn. Near the entrance you will find an elf named Eleyas, who wishes to talk to you. Eleyas works for Iorveth and he has a job for you. Interrogate him for additional information and you will start With Flickering Heart side quest.
After you get the job, head inside the inn and find Zoltan. He is sitting at a table along with other dwarves. If you talk to them, they will offer information on the mines you have to investigate and they will help you get the immortelle.
Note: Dont leave the city without some cat potions and bombs.
When you are ready to begin the quest, leave the inn and head east to find the alderman Cecil Burdon. Talk to him and he will open the shaft to the mines. Additionally, you will start a new side-quest called Hey, Works on in the Mines!
Follow the doors and get inside the mine. While tracking Subterranean Life, if you open your map, you will clearly see the location of the immortelle you are looking for. Its on the eastern side of the mine; however, since you already started Hey, Works on in the Mines! you also have to focus on it.
Your first enemies are some Rotfiends that appear near the entrance.
Combat Tip: Make use of your Igni Sing to defeat the Rotfiends.
After you kill them, follow the first corridor to the south (you should see a dead end). At the end of the tunnel, you will find a corpse guarded by more Rotfiends. Loot the corpse to get the Upper Shaft Key, Balins First Map and Balins Journal 1, then, behind the corpse, look for a tunnel. If you approach it, you can collapse it (Hey, Works on in the Mines!).
Note: At this point you should fully explore the mines and find two more passages. Destroy them to complete Hey, Works on in the Mines!
The final passage you have to collapse for the said side-quest, is near the item you are looking for, but it is well protected by a new creature called Bullvore.
The monsters weak spot is its speed. It is also vulnerable to poison and explosions. If you kill a Rotfiend near the Bullvore, the poison will affect everyone nearby. So, make sure that you keep a fair distance from the Rotfiend, while the Bullvore remains in area.
Geralts Yrden Sign and several bombs will also help you defeat the Bullvore faster, or you can opt to stun him and hit him with your silver sword.
No matter what strategy you decide to use, make sure you loot his corpse for a trophy, and dont leave the area without collapsing the final passage (near the immortelle bushes). Now, you can continue to explore the mine for additional items, or exit using the same path you used to get inside.
Exploration Tip: If you wish to track your progress through the mine, you should light all torches on the walls. This way, you will know which area you have explored.