June 11, 2005 — -- Daniel Tammet of England can verbally reel off the number pi to 22,500 decimal places in just over five hours -- though he admitted after a recent demostration that it made him "very tired."
Tammet, 26, is a phenomenon. He has done lots of amazing things -- like learning Icelandic, one of the world's most difficult languages, in just seven days.
That's because Tammet is an autistic savant. His extraordinary abilities stem from a combination of autism and a condition known as synesthesia. His form of autism, however, leaves him with less limited verbal skills than many other autistics.
Tammet experiences things through a mixing of the senses that the rest of us can't imagine. For instance, when he does math, he said, "I see landscapes in my mind. The numbers turn to shapes.
"They knit together in a way that forms almost like hills and mountains in my mind," he added, "full of color and full of shape and full of movement."
Tammet's talents are like those of the "Rain Man," portrayed on film by Dustin Hoffman and based on the life of Kim Peek, who Tammet once met.
"Amazing," Tammet said of the meeting. "There was something that was special for both of us, and I know it hasn't left me."
There are perhaps fewer than 50 autistic savants in the world, according to estimates by experts. Those few are people with remarkable, often staggering skills and challenges.
Autism may be the fastest-growing developmental disability, according to numbers from the Autism Society of America. Approximately one in 250 children have some form of it. That's up 172 percent in the 1990s.
Some of those kids also have savant abilities. No one knows why.
For all his remarkable gifts, Tammet has some everyday difficulties stemming from his autism. For instance, he doesn't like to come to a beach just a few minutes from his home because it is made up of pebbles — too many even for him to count. That makes him uncomfortable.
Tammet can't drive or do many other things that quire basic coordination. Just walking is something he had to do through an effort of will.
"I had to teach myself how to look and how to walk," he said, "how to move myself, how to coordinate myself without falling over, without looking down, without getting absorbed in my own self, my own world."
Tammet grew up one of nine children in working-class East London. He went to high school and some college, did not get special grades and works primarily as a tutor and consultant because he has a difficult time in a normal work environment.
After years of effort, Tammet has overcome many of his autistic disabilities. Now living outside of London, not only can he relate to people, he can describe what the experience of autism is like from the inside.
He loves silence, for instance.
"I experience it as like a silvery texture around my head, like condensation running down a window," he said. "If there's a sudden noise, it's like a shattering of that feeling."
Such eloquence may be Tammet's most remarkable gift, and it makes him a prime subject for autism researchers.
"Part of what we might learn from studying Daniel is, for example, how he perceives the world," said Simon Baron-Cohen, a professor at Cambridge University's Autism Research Center. "We know that people with autism attend to details much more than most people. And that means that if you're trying to teach somebody with autism, the details will matter."
Researchers from around the world are studying Tammet. Some believe his case may show that there's a savant in all of us, a little Rain Man, you might say, if only we could find a chemical or other way to unlock those abilities.
In a way, one might say Tammet has come back from the country of autism, which is a very difficult place for researchers and for parents to reach.
"I've come from a place where I felt so lonely, and so unwanted in a way," Tammet said. "And I've come along this road, and I've found this bridge, and I've come across it. And I don't know how, I don't know why, but I'm here and I'm able to talk to you today. And, for me, that's amazing."
ABC News' Terry Moran and Lenny Bourin originally reported this story June 5, 2005, on "World News Tonight."