… and south at Widow's Rock. Look for the stone pipe with the steel bars. The aquifer wasn't built by the High Elves, but is a leftover from the ancient Colovian city that once dominated the coastline.
When you get to the opening, take a whiff … the aquifer is still in use, even though most of the High Elves don't know where their refuse is going. I'd bet moon sugar to sea salt no one's been down there in ages. If you can find out where the sewer opens up inside Haven, you've got a surefire way inside.
Explore the area thoroughly. I don't expect you'll see any guards down there, but who knows what's moved in during the years of abandonment? There're hoarvors in the swamps to the north of the city. Wouldn't be surprised if some made a nest down here. Still, those candles I gave you should see them off. Trader I got them from assured me they hate their smell. You'll be safe enough.