A pressure vessel for transporting compressed gas at very high pressure without failing includes an inverted neck at the head so that pressure from compressed gas is applied to the neck in a converging manner. A plug valve is inserted into the opening defined by the neck to seal the pressure vessel. As pressure from the compressed gas enclosed in the pressure vessel increases, engagement between the neck and the plug also increases. Therefore, the pressure vessel can withstand higher levels of pressure from compressed gas before failing during hydro and burst testing.