A pierceable stopper for sealing a container opening, the stopper comprising: a hollow plug portion defining a central axis; a disk portion covering the hollow plug portion, the disk portion concentrically arranged with the hollow plug portion about the central axis and defining a flange extending radially beyond an outside diameter of the hollow plug portion; an internal cavity of the hollow plug portion intersecting the disk portion such that the disk portion comprises a reduced thickness portion over the internal cavity; wherein the internal cavity includes an open end, a cylindrical portion and an undercut portion, the undercut portion extending radially into the flange; a central area of the disk portion defining a pierceable membrane over the reduced thickness portion; a plurality of protrusions extending from a top surface of the disk portion in a first radial zone of the top surface over the reduced thickness portion and in a direction opposite the internal cavity, the first radial zone outside of and concentric with the central area of the disk portion; a second radial zone of the top surface defining a planar ring area outside of the first radial zone; the pierceable stopper being characterized in that the diameter of the internal cavity at its open end and the diameter of the internal cavity at the cylindrical portion are smaller than the maximum diameter of the internal cavity at the undercut portion. a hollow plug portion defining a central axis; a disk portion covering the hollow plug portion, the disk portion concentrically arranged with the hollow plug portion about the central axis and defining a flange extending radially beyond an outside diameter of the hollow plug portion; the disk portion concentrically arranged with the hollow plug portion about the central axis and defining a flange extending radially beyond an outside diameter of the hollow plug portion; an internal cavity of the hollow plug portion intersecting the disk portion such that the disk portion comprises a reduced thickness portion over the internal cavity; wherein the internal cavity includes an open end, a cylindrical portion and an undercut portion, the undercut portion extending radially into the flange; intersecting the disk portion such that the disk portion comprises a reduced thickness portion over the internal cavity; wherein the internal cavity includes an open end, a cylindrical portion and an undercut portion, the undercut portion extending radially into the flange; a central area of the disk portion defining a pierceable membrane over the reduced thickness portion; a plurality of protrusions extending from a top surface of the disk portion in a first radial zone of the top surface over the reduced thickness portion and in a direction opposite the internal cavity, the first radial zone outside of and concentric with the central area of the disk portion; a second radial zone of the top surface defining a planar ring area outside of the first radial zone; the pierceable stopper being characterized in that the diameter of the internal cavity at its open end and the diameter of the internal cavity at the cylindrical portion are smaller than the maximum diameter of the internal cavity at the undercut portion.