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 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 panel holder for holding each said panel in said upright home position, the panel holder comprising, for each said panel, a moveable member (242; 244) and a non-moveable member (235; 235'), one of the moveable member (242; 244) and the non-moveable member being carried by the panel on said top side of the panel and the other of the moveable member (242; 244) and the non-moveable member (235; 235') being translatably suspended below said suspension member, said moveable member (242; 244) capturing and holding said non-moveable member (235; 235') when said panel is rotated upwardly to said home position, and a panel releaser (246, 252) for said panel holder, comprising linkage connected to said moveable or non-moveable panel holder member suspended by said suspension member (242; 244), said linkage being manually moveable relative to said suspension member (242; 244) to translate said suspended moveable or non-moveable member to release the panel from said upright home position and allow said panel to gravitationally rotationally fall to said lower 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 panel holder for holding each said panel in said upright home position, the panel holder comprising, for each said panel, a moveable member (242; 244) and a non-moveable member (235; 235'), one of the moveable member (242; 244) and the non-moveable member being carried by the panel on said top side of the panel and the other of the moveable member (242; 244) and the non-moveable member (235; 235') being translatably suspended below said suspension member, said moveable member (242; 244) capturing and holding said non-moveable member (235; 235') when said panel is rotated upwardly to said home position, and a panel releaser (246, 252) for said panel holder, comprising linkage connected to said moveable or non-moveable panel holder member suspended by said suspension member (242; 244), said linkage being manually moveable relative to said suspension member (242; 244) to translate said suspended moveable or non-moveable member to release the panel from said upright home position and allow said panel to gravitationally rotationally fall to said lower passage closing position.