Across the country, large underground tunnels are commonly used to prevent combined sewer overflows (CSOs). Some of the larger projects have been taken on in Chicago, Atlanta, Milwaukee, Austin, Boston, Cleveland, Houston, Detroit, Los Angeles, New York, Minneapolis, and San Francisco. A typical tunnel project ranges from 7 feet to 33 feet in diameter and from 2 miles to 33 miles in length. The Chicago Tunnel and Reservoir Plan (TARP) is considered the most successful application of this technology. It covers 109 miles and typical tunnel diameters reach 33 feet. Since 1984, when the first TARP tunnels were built, the number of fish species in the Calumet and Chicago River systems have nearly doubled from 33 to 63.

What needs to be put into perspective here is the cost. Combined sewer systems are an infrastructural relic – many dating back to the beginning part of the last century. The amount of new infrastructure to deal with this problem, while successful in reducing CSO discharges to our nation's surface waters, is astronomical in size and cost. Take a smaller project like that in the City of Milwaukee. Over 2 billion dollars was spent on developing those CSO tunnels – more than half of that money was from federal grants. Another thing to consider is this, while the systems are typically able to successfully store and treat the overflows, the massive amounts of energy to make this happen begs quantification.

Is a massive concrete hole in the ground the answer? It is working, but is it the answer?