The San Juan Mountains, in southwest Colorado, are the most extensive mountainous region in the state - an ocean of mountains that extends as far as the eye can see. Some notable San Juan towns are Telluride and Ouray on the northern side, Silverton in the middle, and Durango to the south.
To better understand the nature of the San Juans as a complex mountain range, see Aaron Johnson's excellent San Juans page on Summitpost.org.