They're known for building some of the most advanced technologies known to man, and for years, all of the engineering, science, and technology, for the most part, has been locked up in Black Budget programs that have absolutely no oversight from Congress. This is according to a 1997 U.S Senate report. This means that nobody, not even those who have 'high level' positions within the military, political, and intelligence communities, knows exactly what's going on.
Many of these programs are completely unacknowledged, and the United States has had a history of government agencies existing in secret for years. While the National Security Agency (NSA) was founded in 1952, its existence was hidden until the mid 1960s. Even more secretive is the National Reconnaissance Office, which was founded in 1960 but remained completely secret for 30 years, along with all of the technology it was using.
You can read a more in-depth article on the Black Budget here.
The same type of secrecy surrounds our space program, and what's now known as The Secret Space program. Who knows what's really going on up there.
"It is ironic that the U.S. would begin a devastating war, allegedly in search of weapons of mass destruction, when the most worrisome developments in this field are occurring in your own backyard. It is ironic that the U.S. should be fighting monstrously expensive wars in Iraq and Afghanistan, allegedly to bring democracy to those countries, when it itself can no longer claim to be called a democracy, when trillions, and I mean thousands of billions of dollars have been spent on projects about which both the Congress and the Commander in Chief have been kept deliberately in the dark."
– Former Canadian Defence Minister Paul Hellyer (source