After Tony Ressler left a meeting about a decade ago to complete his $730 million acquisition of the Atlanta Hawks, he saw a vast subterranean expanse near the CNN Center known as the Gulch.

“What is that?” he asked the team’s chief executive, Steve Koonin.

The Gulch, Mr. Koonin explained, was the original center of Atlanta that was first named Terminus because it was the end of the rail line for Western and Atlantic Railroad trains. As America’s love affair with cars took off, parking lots were constructed in the Gulch and roads were built above and around it.