Apparatus for allowing ventilation through a ventilation shaft to an underground ventilation duct fluidly communicating through the ventilation shaft to an atmospheric opening of the shaft and on threat of flooding operable to prevent downward flow of surface water into the underground ventilation duct, comprising a support comprising opposed lateral sidewalls for arrangement in said shaft defining a passage between top and bottom openings of the support for fluid communication of said ventilation duct up through said support to said atmospheric opening, one or more panels (234; 236) having proximal and distal ends (234a; 234b; 236a; 236b), a top side and a bottom side, said proximal end connecting with a horizontal hinge having a hinge axis perpendicular to said opposed lateral sidewalls for rotation of each said panel (234; 236) upwardly to an upright home position not obstructing said passage and rotation from said home position downwardly by gravitational impetus of its own weight to reach a lower passage closing position, said one or more panels having a profile that closes said passage when each said panel gravitationally rotates to said passage closing position, a suspension member (242; 244) unobstructively horizontally spanning said passage supported on said opposed lateral sidewalls (224a 224b) proximate said top opening and holding said one or more hinge connected panels (234; 236) in said passage proximate said bottom opening, a panel handle on said top side of said one or more panels (234; 236) for manually rotationally raising said one or more panels from said passage closing position toward said home position, and a lift arm for each panel, each lift arm having a proximal end pivotally connected by pivot pin to said bottom side of a said panel (234; 236) on a pivotation axis parallel to said hinge axis and a distal end (263a; 263b), each said lift arm being of dimension to contact said distal end with one of said opposed lateral sidewalls when said distal end (263a; 263b) is pivoted upward inside said sidewall for exertion of lateral force onto the connected panel being raised with said panel handle to complete rotation of said panel into said home position. a support comprising opposed lateral sidewalls for arrangement in said shaft defining a passage between top and bottom openings of the support for fluid communication of said ventilation duct up through said support to said atmospheric opening, one or more panels (234; 236) having proximal and distal ends (234a; 234b; 236a; 236b), a top side and a bottom side, said proximal end connecting with a horizontal hinge having a hinge axis perpendicular to said opposed lateral sidewalls for rotation of each said panel (234; 236) upwardly to an upright home position not obstructing said passage and rotation from said home position downwardly by gravitational impetus of its own weight to reach a lower passage closing position, said one or more panels having a profile that closes said passage when each said panel gravitationally rotates to said passage closing position, a suspension member (242; 244) unobstructively horizontally spanning said passage supported on said opposed lateral sidewalls (224a 224b) proximate said top opening and holding said one or more hinge connected panels (234; 236) in said passage proximate said bottom opening, a panel handle on said top side of said one or more panels (234; 236) for manually rotationally raising said one or more panels from said passage closing position toward said home position, and a lift arm for each panel, each lift arm having a proximal end pivotally connected by pivot pin to said bottom side of a said panel (234; 236) on a pivotation axis parallel to said hinge axis and a distal end (263a; 263b), each said lift arm being of dimension to contact said distal end with one of said opposed lateral sidewalls when said distal end (263a; 263b) is pivoted upward inside said sidewall for exertion of lateral force onto the connected panel being raised with said panel handle to complete rotation of said panel into said home position.