I still think the scariest unexplained case he's covered (from what I've seen) was the Khamar Daban Incident. Imagine just having a normal hiking trip when suddenly everyone in your group besides you suddenly drops to the ground screaming, bleeding from every orifice on the head, convulsing, and trying to bash their own heads in all within the span of like five minutes. And then once the carnage is finally over, you're alone in the wilderness, miles from help, with no idea if what overcame your companions will happen to you, too. That's some shit straight out of a Lovecraft novel.
(it was totally residual nerve agents from the Soviet testing years stirred up by the rainstorm, you cannot convince me otherwise)