The Fifty-Niners trace to 1859, during the Colorado Gold Rush. They streamed into Pike's Peak Country, in what would later become the American state of Colorado, in search of gold.
No gold was anywhere close to Pike's Peak, but it was the first visible landmark. There is no count of how many prospective miners died en route to Pike's Peak, but the casualties were appalling.
They gave rise to the slogan, "Pike's Peak or Bust ."
See also: Forty-Niner