The idea of the mono-wheel, both powered and unpowered, has been around for a very long time. Documented examples popped up in the mid 19th century but few, if any, approach the monumental proportions of the one in this photo that we found floating around in cyberspace. More info and pics at www.theoldmotor.com.
The product of the fertile imagination of a Professor E. J. Christie, this gyroscopic unicycle was supposedly capable of speeds of up to 400 miles per hour, although at the time of an article from the April, 1923 issue of Popular Science (above, center), it had yet to be tested. This monster was 14 feet tall, weighed 2,400 pounds and used what appears to us to be a Curtiss OX-5 airplane engine for power.