A manhole inspection unit for vats or tanks comprising: - a frame at least partially housable inside a through opening of a wall of the vat or tank; said frame has a peripheral undercut protruding towards the inside of the frame and defining at least partially a passage opening between an inner environment and an environment outside the vat or tank; - a door hinged to said frame and reversibly movable between a closing position, in which it is substantially abutting against an abutment portion of said frame, and an opening position; in use, said door being arrangeable at least partially inside the vat or tank and said abutment portion can at least partially face an environment inside said vat or tank; - a locking device connected or connectable to said door and activatable on a stop portion of said frame to generate a clamping force adapted to push said door against said abutment portion, said stop portion being, in use, outside the vat or tank; and wherein said undercut defines at least partially said abutment portion and/or said stop portion; wherein said locking device comprises: - at least one locking body activatable on said stop portion; - at least one transmission body constrained or constrainable to said door, said transmission body being connected or connectable to said locking body to transmit said clamping force to said door; - clamping means activatable between said at least one locking body and said at least one transmission body to generate said clamping force; characterised in that the inspection unit comprises a plurality of locking bodies and a plurality of transmission bodies; said clamping means being at least partially activatable between said plurality of locking bodies and said plurality of transmission bodies to generate said clamping force. - a frame at least partially housable inside a through opening of a wall of the vat or tank; said frame has a peripheral undercut protruding towards the inside of the frame and defining at least partially a passage opening between an inner environment and an environment outside the vat or tank; - a door hinged to said frame and reversibly movable between a closing position, in which it is substantially abutting against an abutment portion of said frame, and an opening position; in use, said door being arrangeable at least partially inside the vat or tank and said abutment portion can at least partially face an environment inside said vat or tank; - a locking device connected or connectable to said door and activatable on a stop portion of said frame to generate a clamping force adapted to push said door against said abutment portion, said stop portion being, in use, outside the vat or tank; and wherein said undercut defines at least partially said abutment portion and/or said stop portion; wherein said locking device comprises: - at least one locking body activatable on said stop portion; - at least one transmission body constrained or constrainable to said door, said transmission body being connected or connectable to said locking body to transmit said clamping force to said door; - clamping means activatable between said at least one locking body and said at least one transmission body to generate said clamping force; - at least one locking body activatable on said stop portion; - at least one transmission body constrained or constrainable to said door, said transmission body being connected or connectable to said locking body to transmit said clamping force to said door; - clamping means activatable between said at least one locking body and said at least one transmission body to generate said clamping force; characterised in that the inspection unit comprises a plurality of locking bodies and a plurality of transmission bodies; said clamping means being at least partially activatable between said plurality of locking bodies and said plurality of transmission bodies to generate said clamping force.