Blackreach: Under Alftand
Once you beat the Dwarven Centurion located at the end of the ruins of Alftand, you will come across a table like structure that has an activate command hovering over it. You CAN NOT activate the stand with out the sphere structure that you get from Septimus Signus. He also gives you a blank lexicon for when you're on the Elder Scroll quest line. Upon reaching Blackreach, directly in front of you will be Sinderion's workshop. Now, if you've played oblivion before, you will recognize the name Sinderion because he gave you the quest of gathering nirnroot for his experiments. It seems that even in death he has you gathering nirnroot but this time its a special breed of nirnroot called Crimson Nirnroot. After searching his workshop you will find his journal in which you get a quest where you find 30 Crimson Nirnroot all throughout Blackreach. The journal will explain what to do with them. Before you go searching for the nirnroot, know this, I spent three or more hours searching for them and only came up with 29.
After leaving his workshop you go about exploring Blackreach, if you so choose. Falmer are all over the place, along with frostbite spiders, and Dwarven Centurions. However, the Centurions will not be a problem as long as you don't pull the lever next to them. There are many buildings you can search while in Blackreach, most of which are booby trapped or infested with enemies. Once you reach the tower of Mzark, at the very top you will find about four or five stands of buttons. First, place the blank lexicon in the receptacle, then push the button on the far right continuously until the next button turns blue, do this foreache button until the elderscroll is revealed. Take it from the stand along with the lexicon which is no longer blank and go into the lift which is right below the stairs with the buttons. It will take you back out to skyrim, significantly farther away from Alftand.