The Atlanta Sewers were built in the 1920's in order to prevent any flooding that may happen.
After the Apocalypse, the sewer system was presumebly abandoned; however it's possible that some people pay have sought refuge here due to there being some walkers inside the sewers.
With the streets no longer safe, Rick suggests they try to escape underground. Jacqui, who formerly worked in the Atlanta zoning office, mentions that the old building might have a flood tunnel that would provide access to the sewers, prompting the group to head back downstairs to the basement. T-Dog stays behind to keep an eye on Merle, and to continue attempting to make contact with the rest of the group outside of Atlanta.
In the basement, Glenn, Rick, Andrea, Morales, and Jacqui stand at the top of a ladder that descends into darkness. Glenn infers that they all seem to expect him to be the one to explore the depths and concocts a plan to send Rick and Andrea to watch the storefront, Jacqui to stand atop the ladder to warn of danger, and Morales to come with him into the sewer.
Glenn and Morales travel through the sewer until they reach a grated barrier. They discuss ways to cut through, but they soon abandon the idea when they see a walker devouring a rat on the other side. The walker reaches at them through the grate and they step back.
Glenn, Morales, and Jacqui arrive to report that the sewer is not an option.