In the Bloodshot Stronghold area, there is a whirlpool room. To your immediate left as you enter the room there is a locked door.
How do you open the door?
Possible spoiler in the image of the door (my current quest) and my current progress in the spoiler tag below.
I have gone through the stronghold and Roland was just taken away. I defeated the robots in the immediate area that aggro'd me but I did not enter that last section. I backtracked to here thinking it may have opened it, but nadda. I also opened the cell doors and turned off the electricity in front of the one chest.
In the image above you see the path you walk down to stand in front of the whirlpool and the door on the left (as you face the whirlpool)
Best Answer
There's a side quest you can do for Tannis called Splinter Group. It involves you taking out a group of mutants in the sewers. You cannot open that door before killing them, as their area is locked until you can convince them to open it during the side quest. At the end of their little area is a ladder. It leads up to that door. That's the only way to open it.
On the plus side, XP and two weapons chests!