BOSTON -- Big Dig officials celebrated a landmark in the project Monday at the same time an embarrassing flaw hung over their heads.

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Watch Jack Harper's Report

NewsCenter 5's Jack Harper reports that officials took the media on a tour of the 1.5-mile-long tunnel 120 feet underground that will be the new, depressed Central Artery in about two years.

For the first time, the tunnel between Kneeland Street and the new Zakim Bridge can be walked.

"If you look up at all the green steel, that's all the existing elevated I-93, and all of the brown steel is temporary steel that was put in place in order to transfer the load of the elevated artery onto these slurry walls that we put in place," Mass. Turnpike chairman Richard Kapka said.

The bridge, the most visible sign of the Big Dig at this point, was the source of embarrassment for project officials, when it was revealed that a flaw in the construction of the bridge has weakened it. Air spaces in steel reinforcing rods placed too close together in the concrete will have to be fixed.

"We've done some testing and found that there are indeed voids, airspace in the concrete," project director Michael Lewis said. "We will find out the extent of it, then replace that concrete with solid concrete and reinforcing steel."

The architect is flying into Boston to meet with engineers, do more testing and figure out what has to be done. Only a 10-foot section at one end and maybe at the other end of bridge is affected. Acting Gov. Jane Swift said that the contractors who built the bridge will be responsible for the cost of fixing it.