def tank_geometry(geometry, om_key):
    """
    Returns the IceTop tank geometry object corresponding to a given DOM.
    """
    if om_key.om == 61 or om_key.om == 62:
        return geometry.stationgeo[om_key.string][0]
    elif om_key.om == 63 or om_key.om == 64:
        return geometry.stationgeo[om_key.string][1]
    return None