The unicycle is believed to have emerged in the late 1800's and has now expanded into several varieties. It has moved out of the circus ring and out on the open roads and campuses of America.
They have always required a greater degree of balence than a traditional bicycle and depending on the wheel size, they average from 3 mph to 22 mph. Recent developements in design have expanded the interest in them for more than just performing for a crowd. One of the fastest growing form of unicycling is called MUni or mountain unicycling.
I have enough trouble keeping a bike on a rough trail, trying to keep a seat on just one wheel........... seems crazy to me.
For a view of MUni on video, go here and try and enjoy the scenery.