The aorta is the largest artery in the body, channeling all the blood pumped by the heart through the chest up toward the neck. Branch arteries lead to the head & each arm, while the aorta curves down the chest and abdomen to around the belly-button, where it divides into two narrower arteries to each leg (see Diagram — Circulatory System).
An aneurysm is a ballooning out of an artery if the wall is weak. The danger is that it can eventually rupture, which then carries a high mortality rate (almost 100% without surgery).
Pain virtually only occurs once the artery ruptures. It’s severe, often described as “tearing” (“tear” rhymes with “hair,” not “here”), and may be felt in the back. If there’s a complete rupture, pain begins immediately; if bleeding occurs into the wall of the artery (“dissection”), it may evolve a little more gradually.
Diagnosis is sometimes made by chest x-ray, but more easily & certainly by a chest CT scan. Most cases require surgery.