The diaphragm is a large flat muscle that separates the chest and abdominal cavities. Normally the oesophagus passes through a hole in the diaphragm from the chest into the abdomen where the stomach is located. This hole is called the hiatus. A hiatus hernia develops when this hole enlarges and the usual connections between the diaphragm and the oesophagus break down. In time, this allows the upper part of the stomach to push through or herniate up into the chest. The valve mechanism at the bottom of the oesophagus may then be caused to fail. However, it is important to realise that many people will develop reflux without a hiatus hernia and others may have a hiatus hernia but experience no reflux.