Closing system, comprising: - a tool which can be inserted into the opening of a plate part, - a closing device for closing a hole in the ground, said closing device comprising: - a lightweight plate part (4, 104, 204, 304, 404) for closing the hole in an edge (12, 112, 212, 313, 412) thereof; - locking means for locking and unlocking the plate part relative to the edge, wherein the locking means comprise a mechanism which can be operated via an opening (20, 120, 220, 320, 420) in the plate part (4, 104, 204, 304, 404); - wherein the opening (20, 120, 220, 320, 420) in the plate part (4, 104, 204, 304, 404) comprises a non-round form, and the tool which can be inserted into the non-round opening in the plate part is rotatable between a first insertion position, in which the tool can be inserted into and taken out of the opening, and a second handling position; and - wherein in said second handling position the tool urges the locking means into an unlocked position, and removal of the tool through the non-round opening is prevented; and characterized in that the tool operates the locking means such that: - the locking means are situated at all times in a locked position of the plate part (4, 104, 204, 304, 404) relative to the edge (12, 112, 212, 313, 412) when the tool is situated in said insertion position where it is removable from the opening; and - the locking means are situated in an unlocked position of the plate part relative to the edge (12, 112, 212, 313, 412) only when the tool is situated in said handling position for handling the plate part (4, 104, 204, 304, 404) in the opening (20, 120, 220, 320, 420) of the plate part. - a tool which can be inserted into the opening of a plate part, - a closing device for closing a hole in the ground, said closing device comprising: - a lightweight plate part (4, 104, 204, 304, 404) for closing the hole in an edge (12, 112, 212, 313, 412) thereof; - locking means for locking and unlocking the plate part relative to the edge, wherein the locking means comprise a mechanism which can be operated via an opening (20, 120, 220, 320, 420) in the plate part (4, 104, 204, 304, 404); - wherein the opening (20, 120, 220, 320, 420) in the plate part (4, 104, 204, 304, 404) comprises a non-round form, and the tool which can be inserted into the non-round opening in the plate part is rotatable between a first insertion position, in which the tool can be inserted into and taken out of the opening, and a second handling position; and - wherein in said second handling position the tool urges the locking means into an unlocked position, and removal of the tool through the non-round opening is prevented; and - a lightweight plate part (4, 104, 204, 304, 404) for closing the hole in an edge (12, 112, 212, 313, 412) thereof; - locking means for locking and unlocking the plate part relative to the edge, wherein the locking means comprise a mechanism which can be operated via an opening (20, 120, 220, 320, 420) in the plate part (4, 104, 204, 304, 404); - wherein the opening (20, 120, 220, 320, 420) in the plate part (4, 104, 204, 304, 404) comprises a non-round form, and the tool which can be inserted into the non-round opening in the plate part is rotatable between a first insertion position, in which the tool can be inserted into and taken out of the opening, and a second handling position; and - wherein in said second handling position the tool urges the locking means into an unlocked position, and removal of the tool through the non-round opening is prevented; and - the locking means are situated at all times in a locked position of the plate part (4, 104, 204, 304, 404) relative to the edge (12, 112, 212, 313, 412) when the tool is situated in said insertion position where it is removable from the opening; and - the locking means are situated in an unlocked position of the plate part relative to the edge (12, 112, 212, 313, 412) only when the tool is situated in said handling position for handling the plate part (4, 104, 204, 304, 404) in the opening (20, 120, 220, 320, 420) of the plate part.