The paper demonstrates that a monolayer graphene membrane is impermeable to standard gases, including helium. By applying pressure differences across the membrane, the elastic constants and mass of a single layer of graphene are measured. The pressurized graphene membrane, the world's thinnest balloon, serves as a unique separation barrier between two distinct regions, only one atom thick. The results show that single atomic sheets can be integrated with microfabricated structures to create a new class of atomic-scale membrane-based devices. The impermeability of the graphene membrane allows for controlled strain and tuning of the resonance frequency, enabling the measurement of the mass and elastic properties of graphene. The findings have implications for various applications, including pressure sensors, ultrafiltration, and STM imaging.The paper demonstrates that a monolayer graphene membrane is impermeable to standard gases, including helium. By applying pressure differences across the membrane, the elastic constants and mass of a single layer of graphene are measured. The pressurized graphene membrane, the world's thinnest balloon, serves as a unique separation barrier between two distinct regions, only one atom thick. The results show that single atomic sheets can be integrated with microfabricated structures to create a new class of atomic-scale membrane-based devices. The impermeability of the graphene membrane allows for controlled strain and tuning of the resonance frequency, enabling the measurement of the mass and elastic properties of graphene. The findings have implications for various applications, including pressure sensors, ultrafiltration, and STM imaging.