Do you mean sewer or tunnel entrance? A sewer entrance will be a normal sewer gateway, but the tunnels in faultline are under the map.
@Golden Girl
City of Heroes comics and artwork
I meant the tunnel entrance. The entrance to my mission is off the map.
Freedom. I am playing Jalen Highcloud.
Ok, give me a moment to find a phone booth to change in, and I'll come rescue you 
@Golden Girl
City of Heroes comics and artwork
Thanks a lot for your help.
And for those who need the solution when GG isn't around.
Go to the top of lift H. Just North of the lift you will see a dead tree. There is a slight depression in the ground next to the tree and in that depression is a tunnel entrance. There is also an entrance on the other side of the dam (which is the one most people will give you) but unless you have the Arachnos costume power or some invisibility ability then that entrance is pretty much suicide at the level you get these mission. However if you want to commit suicide then exit the opposite side of the dam. Turn right and in the Arachnos facility you see there is a tunnel entrance.
Don't count your weasels before they pop dink!
That was an awesome thing to do Golden Girl. Love this game.
Enjoy your day please.
Red arrows are entrances to the tunnel, green to the dam.
Map with tunnel markings available as an in-game add-on from Vidiotmaps.
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Might also have been one of the entrances in the parking garage area. If you follow the road to the west when you enter Faultline (which should be to the right, as you are facing south when you enter Fautline), that road will go down to the parking garages. Good to know when you're looking for Vahzilok for the Numina Task Force, too.
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I have been flying around faultine for half an hour trying to find this stupid sewer entrance. I have petitioned for help and have gotten nothing. Any help would be appreciated.