A hiatal hernia happens when the upper part of the stomach bulges through the large muscle that separates the abdomen and the chest. The muscle is called the diaphragm. The diaphragm has a small opening called a hiatus. The tube used for swallowing food, called the esophagus, passes through the hiatus before connecting to the stomach. In a hiatal hernia, the stomach pushes up through that opening and into the chest. A small hiatal hernia usually doesn't cause problems. You may never know you have one unless your healthcare team discovers it when checking for another condition. But a large hiatal hernia can allow food and acid to back up into your esophagus. This can cause heartburn. Self-care measures or medicines can usually relieve these symptoms. A very large hiatal hernia might need surgery.