On the outside, 31-year-old Daniel Tammet is an unremarkable young man. But behind Tammet's bookish exterior lies a superhuman gift: one of the most extraordinary brains our planet has ever seen. He is a mathematical genius, capable of astronomical calculations in the blink of an eye. And he's a gifted linguist, speaking nine languages, including one he created called Manti.  Daniel Tammet explains his unique ability to see numbers as shapes and colors.  Tammet says he was born with the ability to experience numbers in an exceptionally vivid way.  "The numbers are moving in my mind," he says. "Sometimes they're fast, sometimes they're slow. Sometimes they're dark. Sometimes they're bright. That emotion, that motion, that texture will be highly memorable for me."  The phenomenon is called synesthesia, a mixture of the senses that results in a heightened sensory experience. Tammet is able to see and feel numbers. In his mind's eye, every digit from zero to 10,000 is pictured as a 3-dimensional shape with a unique color and texture. For example, he says, the number fifteen is white, yellow, lumpy and round.  Watch the full story on "SuperHumans!" a special edition of "20/20" Tuesday, June 1 at 10 p.m. ET  Synesthesia occurs when regions of the brain associated with different abilities are able to form unusual connections. In most people's brains, the recognition of colors, the ability to manipulate numbers, or language capacity all work differently in separate parts, and the information is generally kept divided to prevent information overload. But in synesthetes, the brain communicates between the regions.  Tammet doesn't need a calculator to solve exponential math problems such as 27 to the 7th power -- that's 27 multiplied by itself seven times -- he'll come up with the answer, 10,460,353,203, in a few seconds.  Tammet visualizes numbers in their unique forms and then melds them together to create a new image for the solution. When asked to multiply 53 by 131, he explains the solution in shapes and textures: "Fifty-three, which is round, very round...and larger at the bottom. Then you've got another number 131, which is longer a little bit like an hourglass. And there's a space that's created in between. That shape is the solution. 6,943!"  Tammet first discovered his mathematical abilities as a child, the eldest of nine children in his family in England.  "I learned to count, like anyone else, at a young age, and when I did I would see colors," he said. "I would see pictures in my mind. I assumed at the time that everyone saw numbers as I did."  Tammet didn't do math as it was taught in school. Instead, the answers just came to him.
Daniel Tammet, one of the world's 50 living autistic savants, shows genius that has astounded researchers. His mental feats include memorizing and reciting 22,514 digits of the math constant pi and learning the Icelandic language in a week.