The mission that introduces the Lost City of Atlantis to CityVille is Make Some Waves, and below you can check out the requirements:
– Expand to Atlantis
– Collect 40 Atlantis Artifacts
– Complete Upgrade a Wonder
Unfortunately, the Lost City of Atlantis doesn’t offer too much (except for, of course, a lot of color in your City): it gives you 500 extra population spots for your population cap and can also provide you with blueprints. But this is a challenge nevertheless and since it will not be available forever, you should do your best to expand to it and claim it until it gets lost again… and this time forever, even in CityVille!