Saturn's moon, Enceladus, may not have a heart of stone—at least, not completely. A new model suggests the satellite has a rubble-filled pile of boulders and ice at its core, rather than a more conventional solid stone center. This “fluffy core” could help solve the mystery of the moon’s underground ocean. A watery layer beneath Enceladus’s crust has long been suspected to exist because of the constant eruption of geysers at its southern pole. But scientists have said that any such ocean should have frozen over the lifetime of the Saturn system. Tidal heating that warms the insides of moons and planets in orbit would simply not be enough to keep this ocean in a liquid state if Enceladus had a solid core. And antifreezing agents such as ammonia would eventually separate from the water if they were present in large enough amounts to prevent ice from forming. But this new model makes antifreeze irrelevant while resurrecting the tidal heating hypothesis: A heart of rubble would flex more easily with the tidal pull of Saturn, emitting enough heat to maintain a liquid layer. And scientists say this is perfectly plausible. If temperatures never climbed high enough to melt the boulders and pebbles at the moon's heart into a single, solid core, it would have remained a rocky rubble pile, extending the liquid lifetime of Enceladus’s underground ocean.