Ever see an inner tube with a bubble/bulge in it? That's what an aneurysm is, a bulging of the blood vessel wall caused by a structural weakness there. You can have them anywhere in your body. When they rupture, especially some of the brain and heart vessel varieties, it's usually LIGHTS OUT. That's what killed John Ritter, a ruptured aortic aneurysm. Since they're symptomless unless/until they rupture they're usually only found before they cause problems when doctors know there's a genetic history of them and they run scans (usually MRI/MRA) or they're looking for something else and happen to run across the aneurysm.