LAS VEGAS (AP) — The U.S. Air Force has warned people against participating in an internet joke suggesting a large crowd of people "storm Area 51," the top-secret Cold War test site in the Nevada desert.

A prank event on Facebook that's attracted more than 1 million interested people suggests that a mass of people attempt to run into the mysterious site at 3 a.m. on Sept. 20.

The site is part of the vast Nevada Test and Training Range and has become the center of UFO conspiracy theories.

The Facebook event jokes "they can't stop all of us" and "Let's see them aliens."

Nellis Air Force Base sai...