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THEY would be a surfer’s dream – if they could ever be ridden. Even the monster rollers off Hawaii cannot match in size and strength the waves that course the bottom of the ocean.

How do they form? Underwater waves start where ocean tides push water over obstacles on the seafloor such as mountains and ridges. This water, which is the densest layer because it is the coldest and the most saline, is then deflected upwards into water that is less dense. It then slows and is ultimately dragged back down by gravity until it overshoots into deeper, denser water –…

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