We present JeL-a bio-responsive immersive installation for interpersonal synchronization through breathing. In JeL, two users are immersed in a virtual underwater environment, where their individual breathing controls the movement of a jellyfish. As users synchronize their breathing, a virtual glass sponge-like structure starts to grow, representing the users' physiological synchrony. JeL explores a novel form of interpersonal interaction in virtual reality that aims to connect users to their physiological state through biofeedback, to each other through physiological synchronization, and to nature through connecting with a jellyfish and collaboratively growing a glass sponge-inspired sculpture. This form of immersive, bio-responsive interaction could ultimately be used to encourage self-awareness, a feeling of connectedness, and consequently pro-social and pro-environmental attitudes. Here, we describe the motivation, inspiration, design elements, and future work involved in bringing this system to fruition.