Complex sentence: Unbeknownst to McCadden, the "mummy" is not the body of a dead Egyptian, but an extraterrestrial alien in suspended animation, being wrapped up and buried alive thousands of years before and covered with a dormant, green fungus.
Split into simpler sentences: Unbeknownst to McCadden, the "mummy" is not the body of a dead Egyptian, but an extraterrestrial alien in suspended animation. It was wrapped up and buried alive thousands of years before, covered with a dormant, green fungus.