The Stone Guardians are massive stones structures that have been empowered by the Heart of Atlantis to create a protective barrier around the city of Atlantis whenever it is threatened by a major cataclysmic event. They are generally only active under such dire of emergencies, otherwise going dormant whenever danger is averted.
Trivia: The Stone Guardians did a recycled animation of clapping just like Julie Weissman did in Kids In Motion.