The aorta is the major artery in the body. It takes blood from the heart through the chest and into the abdomen. All of the major arteries in the body come off of the aorta.
For some reason, not fully understood yet, 97% of all aneurysms of the aorta occur in the abdomen - the Abdominal Aortic Aneurysm (AAA). Only 3% of Aortic Aneurysms occur in the aorta in the chest.

Like any balloon that is getting bigger and bigger - it may pop. Similarly with an abdominal aortic aneurysm, there is no problem until the wall gives way and it bursts - a ruptured aneurysm. If an aneurysm ruptures - only 50% of people survive to get into hospital and 50% of those die despite medical care. Therefore if an Abdominal Aortic Aneurysm is allowed to rupture there is only a 25% (ie: 1 in 4) chance of survival.