Don't know about Area 51, but I do believe. When my husband was in the AF they saw one come in right above their radar site one night. He and his Sgt went out and just stood there and looked up at it. He said all the security cameras had their tapes destroyed, so there was no evidence, so typical of our government.
There's a book called the The Day After Roswell written by Colonel Phillip J. Corso (Ret.)and it is about his account of what he witnessed after aliens crashed in Roswell. He makes claims that he viewed the corpses of aliens and eventually became a top pentagon offical who was involved in reverse engineering. He claims night vision goggles, fiber optics, lasers, computer chips, etc. were reversed engineered with the help of Bell Labs. The Author died a year after the book was released. Fascinating reading, but not sure I buy it.