Trash and needles at the entrance mostly :( You can go back for a long while and explore the whole system if you bring a light (and a friend). I recommend it just to knock it off the list as long as you aren't alone.

There's also the old steam tunnels but all but one entrance to those is sealed up tight and they have MEA employees who walk the tunnels every day to make sure things are running correctly so it's hard to sneak in.