As Las Vegas prepares to celebrate its centennial, another Nevada institution is marking a milestone birthday this month. The world's best-known secret military base, Area 51, turns 50 years old this month.Area 51, located at dry Groom Lake in Lincoln County, has been "the" location of choice for the most classified military programs in the world. For many years, the government would not admit its existence. People who worked there were sworn to secrecy. Now, they're talking. Area 51 wasn't supposed to be a permanent base. It was built in 1955 for the U-2 spy plane. But when that work was finished, other so-called black projects were sent there, and today it is a multi-billion dollar facility that essentially cannot be duplicated. The people who've worked there over the years are justifiably proud about their work in protecting our national security. But they've never been able to talk about it, not to their spouses, not even to each other. Now, the people who were there, and the secrets they kept for so long, are breaking their silence.Buses with blacked-out windows, cameras that scan for any movement, sensors buried in the dirt, armed choppers that patrol the skies are all ominous signs that warn of deadly force. The secrecy that's long been the trademark of Area 51 is as pronounced today as it's ever been.