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Transmission electron microscopes (TEMs) and scanning transmission electron microscopes (STEMs) are powerful tools for imaging on the nanoscale. These microscopes cannot be typically used to image processes taking place in liquid media because liquid simply evaporates in the high-vacuum environment of the microscope. In order to view a liquid sample, it is thus necessary to confine the liquid in a sealed vessel to prevent evaporation. Additionally, the liquid layer must be very thin to minimize electron scattering by the suspending medium. To address these issues, we have developed a flow cell with a height of tens of nanometers, sandwiched between two thin silicon nitride membranes. The cell is equipped with electrodes for actuation and sensing. The cell is thin enough to allow the transmission of electrons and the real-time imaging of nanoparticles suspended in liquid. This paper details the fabrication process, which relies on plasma-activated wafer bonding. Some of the advantages of our nanoaquarium include the thinnest observation chamber of any reported in situ TEM/STEM device, integrated electrodes for sensing and actuation, and wafer-scale processing that allows bulk device production. Device performance was demonstrated by STEM imaging of gold and polystyrene nanoparticles suspended in water with excellent resolution. Potential applications of the device include imaging of colloidal crystal formation, aggregation, nanowire growth, electrochemical deposition, and biological interactions.