I had this dream back in 2002. I remember this because it was about 5 months after the WT attacks in NY. We were watching a video at the skatepark, and a kids dad sees a fullpipe and says hey there's one of those off I-285.
We hurriedly went searching in the area he talked about and then found a giant drainage ditch that had about a 18-20ft tall fullpipe in it. Unfortunately it had about a foot and a half of water running through it. We figured we'd just come back during the dry season, but then one of my buddies got the idea that we should walk through it and see if it led to anymore drainage pipes. We didn't really think too much about where it might lead and decided to go on a trek through it.
This full pipe was probably about a quarter mile long and popped out into a big ditch that had another slightly smaller full pipe on the other side. We walked through the smaller one for probably 3/4's of a mile, until we ran out of light and really counldn't see any further.
So we walked back to the ditch. At which point one of my buddies decides his ankles hurt from walking through the pipe and that we should just climb out of the ditch and walk above ground back to where we parked. We got to about the top of the ditch when we realized that we were somewhere we REALLY shouldn't be... Especially considering the time frame mentioned above. Ran down the ditch and ran back through the pipe and as we were exciting noticed all the security cameras around and got the hell out of dodge as fast as we could. Another friend drove by about a half hour later to see security people all over the place.
If you take a look at google maps and look at Atlanta there's a really big thing down on the bottom near 285. You seriously can't miss it. Put 2 and 2 together.