"One of the first known models of a mountain bike was back in 1896, where bicycles were modified for off road use by the Buffalo Soldiers from Montana to Yellowstone in the United States."
"The largest mountain bike race in the world (by a number of participants) is called the Birkebeinerrittet that is a 95 km (59 miles) race that takes place each year in Norway."
"Mountain biking can be conducted nearly everywhere from the gravel road to a decent backyard."
All the facts are from Here along with more information