A 53-year-old South Florida man surfs with sharks and climbs
mountains while missing a fourth of his brain.  Friends of Chris
Marshall always said he had no fear.  "I suppose my whole life
I've liked to get out and do things," he said. "Growing up in
Australia, surfing, camping, traveling."  An arachnoid cyst
covers his brain's frontal lobe, the area that doctors say
controls fear.  The brain scan pictured below shows the cyst,
the dark section of Marshall's brain, where tissue should be.
Chris Marshall brain scan  "I think it is always amazing to us
when we see someone missing a large part of brain and is
functioning at such a high level," said Dr. Linda Sternau, a
neurosurgeon at Memorial Regional Hospital in Hollywood.  An
arachnoid cyst is a cerebrospinal fluid-filled sac located
between the brain or spinal cord and the arachnoid membrane, one
of the three membranes that cover the brain and spinal cord.
WATCH: Sternau explains arachnoid cysts  Smaller ones typically
go undiagnosed. Doctors believe Marsh