A huge trench down to the void

I was playing in my world today on single player, looking for the stronghold. I was swimming through an ocean when i see up ahead what looks like the ocean just stopping, and void after it. I thought it was an unloaded chunk so I kept swimming, when I suddenly fall into the void. I flew over in creative and looked around and what I saw was a massive trench of void, one chunk wide, stretching far past render distance. I have no idea what caused this, but I was playing in a world with two modes, but nether affected world generation. I had all experimental settings on for my mods, so that may have something to do with it. 
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coords: -1002, 100, -519