A few years ago I was in Guam and could walk out on the reef on the eastern side about 200 yards in water up to my waist. The water is a beautiful turquoise until one gets to the edge of the reef where there is a distinct color change to a dark, almost black, blue. I stopped about 10 feet from this dark line, knowing it was the edge of the 35,000 foot deep Marianas Trench. If I was a swimmer I could have swam out over the abyss. But I'm just a wader. Still an exhilarating experience. In the picture below Guam is the figure "8" shaped dark spot at the southern end of the Marianas Islands chain.