Proof of the secret of death, discovery of it, would mean, imho, conquering it and mastering it.
People who've had Near Death Experiences claim they left this place and returned. I found one most interesting account, where real Scientific Knowledge was supposedly brought back from beyond Death , here :-
"Mellen-Thomas Benedict is an artist who survived a near-death experience in 1982. He was dead for over an hour and a half after dying of cancer. At the time of his death, he rose up out of his body and went into the light. Curious about the universe, he was taken far into the remote depths of existence, and even beyond, into the energetic void of nothingness behind the Big Bang. During his experience, he was able to learn a great deal of information concerning reincarnation. Because of his near-death experience, he was able to bring back scientific discoveries. Mr. Benedict has been closely involved in the mechanics of cellular communication and research dealing with the relationship of light to life called Quantum Biology. This research is providing dramatic new perspectives on how biological systems work. Mr. Benedict has found that living cells can respond very quickly to light stimulation resulting in, among other things, high speed healing. He is a researcher, inventor and lecturer who holds six U.S. patents. "