The following is the final journal entry of cave explorer Ethan Baker. It was found at a pitched camp site on the outskirts of Da Nang, Vietnam, just days after it was written. Ethan has since been declared a missing person.

12th November 2009 - Da Nang, Vietnam

Vietnam is famous for its huge cave systems and their cathedral like features.

It was here where I found one remote cave near the city of Da Nang. As I write I am currently in hiding and to be quite honest I don't think I have long to live. The police won't protect me and my family think I'm crazy, but in that cave a few weeks ago an innocent tourist trip turned into the darkest day of my life.

I finally found a nice part of the cave, past the beautiful waterfalls and cascades in a place deeper than the rest of the cave system.

The locals say that is was this place where many Viet Cong took refuge, but people would be stuck down there for months and resorted to cannibalism. This of course is a myth, I pray. I decided to shout various things because I had never heard such a splendid echo! One of nature's great phenomenons, and for the Ancient Greeks a path into immortality.

I remember shouting three things.

"Hello," I yelled.

I heard the copycat response a few seconds later.

"Is anybody there?" was the next thing I shouted, and the echo was, as expected, my echo.

However, it was then I thought it would be funny if I shouted, "I'm going to kill you" - knowing full well this would be my echo. But the response, I am sure that I received, was, in a voice very similar to my own, and it sounded something like this:

"Không, nếu TAO giết MÀY đầu tiên"

My echo was not received and this was the only thing I heard back, but it had the same reverberating quality of an echo. I decided to say the same words again:

"I'm going to kill you."

"Không, nếu TAO giết MÀY đầu tiên" was the response, this time I could hear it being said a little quicker, but still, for I know it well, in the timbre of my own voice. One last time I said it.

"I'm going to kill you."

"Không, nếu TAO giết MÀY đầu tiên!"

I do not remember shouting it particularly gruffly, but the responding echo sounded very angry and I felt rather petrified. However, because I had said those words and heard the response a few times I memorized the basic sound of the mysterious echo. When I left the cave I asked my tour guide what it meant. He looked at me rather puzzled, quizzing me as to whether I'd met anybody in the caves. I said no, thinking it was totally unrelated. He told me what the words meant in Vietnamese:

"I'm going to kill you."

"Không, nếu TAO giết MÀY đầu tiên." - "Not if I kill you first."

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