. >> reporter: if flippers on dry hand, or a unicycle underwater, it might look like ashrita furman isfor the circus. >> they think i'm crazy. >> reporter: but he may be the most prolific athlete you've never heard of. >> felt great. i want to do it again. i juggled underwater with sharks. i raced against a yak in a sack. and i beat the yak. >> he has, at one time or another, attempted and held 367 guinness world records. >> i don't think of myself as the great guinness record holder. i think of myself as a big kid who is having fun. >> reporter: if you define fun as jumping rope underwater 900 times in 1 hour, or pedaling 1.8 miles below the surface. guinness most famously keeps records for natural wonders. everything from the tallest man, the biggest gold nugget, to the fattest twins. but then, there are those superlati superlatives, or triumphs of the human spirit. >> there's a dichotomy of measurement and chronicling records. but also the slightly off-the-wall, interesting, unexpected world of record-breaking, where you really have to expect the unexpected. >> reporter: no one exem