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COLUMBUS, Ohio (AP) — A massive drill has finished the job of boring a 4.5-mile tunnel hundreds of feet below Columbus in a project aimed at stopping water pollution.

The spinning disc on the front of the 540-foot-long drill capsule emerged through the bottom of a huge concrete shaft connecting it to the surface last week, the Columbus Dispatch reported Saturday (http://bit.ly/1O7JFEa).

“It popped out right where they expected,” said George Zonders, spokesman for the city Department of Public Utilities.

The $371 million project is the most expensive public-works effort in city history, and came in at $29.5 million more than planned, the paper said.

Once completed in 2017, the tunnel project will hold storm water and sewage and keep it from overflowing into the Scioto River during heavy rains.

The tunnel was the city’s response to a U.S. Environmental Protection Agency mandate that the city address wastewater runoff.

Workers have already begun dismantling the giant boring machine with plans to bring it up a little bit at a time.

An odor-reduction facility next to the shaft created by the drill will pull air out of the combined storm and sanitary sewers downtown and filter it, eliminating foul-smelling sewer gas that might seep from the system into neighborhoods.

In 2005, the city said it was proposing a $2.5 billion strategy to reduce overflows, including two other deep tunnels along Alum Creek and the Olentangy River.

Since then, the city has shifted strategies and is preparing to ask the state EPA to revise that plan. Other options are techniques mimicking natural processes to deal with rainwater during heavy storms, such as rain gardens that help the ground absorb the water rather than channeling it through a sewer line.

That approach would allow Columbus to reduce the scale of the other proposed tunnels.

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Information from: The Columbus Dispatch, http://www.dispatch.com