The Flapping Lung Sign, also known as the Jellyfish Sign, is a dynamic sonographic marker of compressive atelectasis in the setting of moderate to large pleural effusion. On B-mode ultrasound, the compressed, non-aerated lung appears as a mobile, echogenic, lobulated tissue mass surrounded by anechoic pleural fluid. With each respiratory cycle, this atelectatic lung moves pendulously, “flapping’’ inward and outward, resembling the rhythmic motion of a jellyfish swimming within the fluid collecti