Brain Aneurysm A brain aneurysm is a weak or thin spot on a blood vessel in the brain that balloons and fills with blood. It is usually located along main arteries on the underside of the brain and the base of the skull. [Please download the PDF file if you are unable to view the image below.] Content serves as a guide only. Seek the advice of your doctor for more details Information correct as of April 2020 Reproduced with permission from the National Neuroscience Institute (NNI)