man: right now, we're at the bottom of the rockdale construction shaft. we're 310 feet below grade, deep under atlanta in hard rock. in the downtown area of atlanta, the sewer system and the stormwater system are combined and there are overflows during storm events, and so the purpose of this system is to relieve that flow, take it into the tunnel, transport it to a brand-new treatment plant, clean up the chattahoochee river. narrator: instead of the combined sewage overflowing into the river, it will flow into this tunnel that acts as a storage tank. the water will then slowly empty into the new plant for treatment before it's released back into the river. man: the system in total is about 8 1/2 miles, 27 feet in diameter. most of the time it will be dry. the only time it will fill is when the sewer system is overwhelmed by the storm. it is a massive project. our budget was $210 million. we've worked about a million and a half man-hours. hunter: it is an incredible amount of work. our capital program right now is $3.9 billion. over a period of less than