Well your guess of the Pacific Ocean would be the closest thing I could think of as never ending. It is just huge, and scattered with tiny islands that are surrounded by miles and miles of sea. To hit one while drifting would be a miracle, and to survive at sea at all in the canoes used to populate them was amazing.
Here are some of the deepest things in this world:
http://www.youtube.com/watch?v=JT-INK5yHLA&feature=related
http://www.youtube.com/watch?v=KGHxVW1NbrQ&feature=related (click on more info in the description)
You could also be thinking of the mariana trench which is the deepest part of the Pacific Ocean.
This is pretty cool too:
http://www.youtube.com/watch?v=Chi3FI381GI&feature=related
You may also be thinking of underwater vents or valcanoes. I didn't find any endless cracks in the ocean floor. Sorry.
Not exactly sure what you are looking for specifically, but it was fun doing this research.:)
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