A duo of astronomers has accomplished a difficult feat: determining the 3-D structure of an interstellar cloud, the birth site of stars. The advance not only reveals the true structure of the molecular cloud Musca, which differs from previous assumptions in looking more like a pancake than a needle, but it could also lead to a better understanding of the evolution of interstellar clouds in general, which will help astronomers answer the longstanding question: What determines the number and kind of stars formed in our Galaxy? Reconstructing the 3-D structure of interstellar clouds has been a major challenge, because astronomical objects can only be observed as 2-D projections on the sky. Determining the 3-D nature of the clouds is critical for a better understanding of the processes occurring within them. Here, Aris Tritsis and Konstantinos Tassis managed to reconstruct the full 3-D structure of Musca thanks to its striations (wispy stripe-like patterns). They show that these striations form by the excitation of fast magnetosonic waves (longitudinal magnetic pressure waves) - the cloud is vibrating, like a bell ringing after it has been struck. By analyzing the frequencies of these waves the authors produce a model of the cloud, showing that Musca is not a long, thin filament as once thought, but rather a vast sheet-like structure. From the 3-D reconstruction, the authors were able to determine the cloud's density. Tritsis and Tassis note that, with its geometry now determined, Musca can be used to test theoretical models of interstellar clouds.