THORNTON, Ill. (CBS) — The “last blast” on Monday at the Thornton Quarry, along I-80, was the first step toward using the big hole to help relieve flooding on the South Side and in the south suburbs.

After a couple of false starts, an explosion ripped tons of 400-million-year-old limestone away from a wall on the southwest side of the Thornton Quarry Monday morning.

READ MORE: Celebrating Black History Makers: Sydney Barber Is First Black Female Brigade Commander At U.S. Naval Academy

After the last of the tons of limestone is mined from the quarry, the Metropolitan Water Reclamation District says the reservoir will be able to hold up to nearly 8 billion gallons of rainwater and raw sewage.

David St. Pierre, executive director of the Metropolitan Water Reclamation District, said computer modeling was done of the 2008 storm that caused massive flooding in the south suburbs, and it found the reservoir would have been able to handle those flood waters.

READ MORE: Trouble Accessing 1099-G Tax Forms For Unemployment Benefits Online? A Tech Expert Has A Possible Fix

John Lemon, principal civil engineer for the project, said when the reservoir is fully connected to the Deep Tunnel project, it’ll mean “more water out of people’s basements” on the city’s South Side, and in the south suburbs, and less polluted water in Chicago River waterways.

Cleaner waterways because of less pollution would mean more economic development, according to Lemon.

St. Pierre said the Thornton Reservoir will be “a significant improvement for flood control. It’s not the total piece, but it’s a large piece.”

MORE NEWS: CBS 2 Exclusive: Now In New Home, West Side Mom Says She Never Left Kids Alone Or Without, Says People Don't Know Her Real Struggle

The Thornton Quarry reservoir should be ready to relieve flooding starting sometime in 2015.