Mountains form the spine of the continent - a great divide separating east from west. The first explorers found themselves in a no-man's-land that was the realm of mountain lions and grizzly bears. But for the mountain men there was more than breathtaking scenery and the lure of furs; there was the prospect of gold and silver to be hewn from the hills. One discovery survived the onslaught to become perhaps the richest treasure of all: the world's first National Park - Yellowstone.