A hiatal hernia happens when the stomach pushes through the hiatus (opening) in the diaphragm, entering the mediastinum, which is the cavity that holds the heart. When you inhale, the hernia becomes more prominent, because the diaphragm contracts. It is also more prominent when swallowing, when the esophagus shortens. That said, these symptoms are minimal so, while they would be noticeable during a medical examination, most people are never aware of the hernia being there at all.