HOLLYWOOD, Fla. -

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.

"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 Marshall was born with the cyst and it grew larger over time because of drastic changes in air pressure when he hiked or climbed mountains.

Marshall's cyst was discovered after he began having tremors in one hand, especially at higher altitudes.

"I didn't know what was causing them," said Marshall. "I had one one year, a couple another year, so I didn't do much, but then I started to have headaches."

Doctors began monitoring Marshall for about a year and a half, but his symptoms became worse. Even small changes in altitude caused headaches.

Sternau opened up the cyst, relieving the pressure and stopping Marshall's headaches.

"You allow the spinal fluid to communicate with the other areas of spinal fluid of the brain," she said.

Marshall traveled to Machu Picchu, Peru, fifteen months later. He later climbed Mount Kilimanjaro headache free.

"It was just an emotional thing," he said. "Here, I was having been through so much. Five years before and now life is normal and I can pretty much do whatever I wanted."