BOSTON -- Another landmark in the $14 billion Big Dig will be reached this month, when the tunnel that is the heart of the massive project will be open for public tours.

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Watch Ed Harding's Report Go Beyond The Big Dig

NewsCenter 5's Ed Harding reported that the tunnel that will replace the Central Artery is nearing completion, and Big Dig officials are opening it for tours on Aug. 25.

"It's 125 feet below street level," Mayor Thomas Menino said. "The public will be truly amazed by that."

Those on the tour will be able to walk the length of the tunnel to South Station and back.

"We're proud of it," spokesman Sean O'Neill said. "Men and women have worked on it for 10 years, and they've a lot to show off, and they want everyone to come down and see it."

The tour will be similar to an event on Mother's Day, when 250,000 people walked across the Zakim Bridge. The tunnel tour will be from noon to 5 p.m. on Aug. 25.

There will also be a major change over the weekend to the commute. The Mass. Pike eastbound off-ramp to South Station and downtown will open in a new, permanent location on Saturday, Aug. 10, at 5 a.m. This new exit provides motorists with direct access to Kneeland Street and Atlantic Avenue.

In addition, Atlantic Avenue northbound, which has been closed for the past four years, will now be open to thru traffic traveling north from Kneeland Street toward Congress Street.

Drivers heading to South Station, South Boston Waterfront or the Financial District should continue straight on the new Atlantic Avenue northbound. Drivers going to Chinatown or the Theater District should turn left at Kneeland Street.