Apparatus for allowing ventilation through a vertical 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 manually 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 proximate said top opening and holding said one or more hinge connected panels (234; 236) in said passage proximate said bottom opening, a drain in at least one of said one or more panels (234; 236) intermediate said proximate and distal ends thereof, said drain comprising a conduit passing through said panel (234; 236), said conduit having an opening at said panel top side and a self actuating drain closure on each said panel having a drain, said closure comprising a plug pivoting rotatable on a drain plug axis positioned between said drain conduit and said distal end of each said panel, said drain plug axis paralleling said horizontal hinge axis, said plug being manually pivotable upwardly on said drain plug axis to remove said plug from said drain opening to drain water contained above said one or more panels when each said panel is in said passage closing position, said plug by gravitational impetus of its own weight automatically pivoting downward to place said plug in said conduit opening when said panel (234; 236) containing said drain is pivotally raised upward to said home position, whereby the conduit opening is already closed when the panel gravitationally rotates downwardly to said passage closing 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 proximate said top opening and holding said one or more hinge connected panels (234; 236) in said passage proximate said bottom opening, a drain in at least one of said one or more panels (234; 236) intermediate said proximate and distal ends thereof, said drain comprising a conduit passing through said panel (234; 236), said conduit having an opening at said panel top side and a self actuating drain closure on each said panel having a drain, said closure comprising a plug pivoting rotatable on a drain plug axis positioned between said drain conduit and said distal end of each said panel, said drain plug axis paralleling said horizontal hinge axis, said plug being manually pivotable upwardly on said drain plug axis to remove said plug from said drain opening to drain water contained above said one or more panels when each said panel is in said passage closing position, said plug by gravitational impetus of its own weight automatically pivoting downward to place said plug in said conduit opening when said panel (234; 236) containing said drain is pivotally raised upward to said home position, whereby the conduit opening is already closed when the panel gravitationally rotates downwardly to said passage closing position.