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As a child, I was always fascinated by the propensity of water going down a drain to form a vortex. I created a world with a global ocean where there was a narrow body of land running north-south which had a tunnel running under the land from one side to the other, connecting the sea on one side to the other (think a "Panama Tunnel"). My vision was that the difference in sea-level from one side to the other would cause water to flow through, and not being on the equator, Coriolis forces would supply the necessary angular momentum to form a vortex.

Is this a valid means by which a vortex big enough to swallow a ship could occur? Are there any other means by which this could be achieved, either created or natural, but requiring nothing more than geography to achieve, i.e. no artificial energy input.

On geological terms, such a feature need not last all that long - a few tens of thousands of years would be plenty.

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No, this is not a valid means. (There is a valid means explained below)

The biggest water movement known with some certainty is the filling of the mediterranean basin through Gibraltar's strait (5.3 million years ago). It moved that mount of water in two years [link] (link in spanish), so it did not last so long.

The key for any permanent water movement is that you somehow put more water upflow and drain it downflow. Why are rivers permanent? Because you have rain in the mountains, which puts water upflow, and evaporation in the ocean, which removes water downflow.

If there is no rain, rivers will dry and disappear. In the imaginary case in which there is no evaporation but there is still rain, sea level will raise more and more until a huge ocean covers everything.

So you can have a gigantic whirlpool if you happen to be lucky enough to have (or if you decide to create, as it is your world) a combination of several factors:

  1. A tunnel at the bottom of a water body. It can not be the Ocean or any big Sea, since you need...

  2. A lower water body to receive the drained water

  3. Low depth over the upper tunnel mouth. Remember that a full bathtub creates almost no whirlpool, and the swirl gets stronger and stronger the lower the water level gets.

  4. Constant supply of water to the upper water body.

As an imaginary example, just to get into some mental image: make it double or triple the flow of Colorado river at Hoover dam (by means of having some more rain upwards) and try to drain all that water through the current hole. You simply can not, so you will need a bigger tunnel. That bigger tunnel below the dam will cause exactly the kind of whirlpool you want.

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One scenario that might cause this is if you had a large inland sea substantially above main sea level with a lot of rivers flowing into it.

The inland sea would drain out through an underwater hole feeding into a large river (some interesting geology would be needed but a soft limestone layer with granite over and some convenient fault lines might do it).

Above that drain into the underwater river would be the massive whirlpool you desire.

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Well your example actually could work with some understanding. There are real vortexes/maelstroms out there though in the real world. All of them however are linked to the tides some examples. I've also seen a special case of this where the tides from one side of an island to the other will draw water through underground tunnels, so depending on the direction of the tide one side is upwelling water and on the other side of the island it is a dangerous vortex, when the tide switches the direction switches and they reverse rolls.

These particular ones are only big enough to be dangerous (on the vortex side) to people and maybe a canoe or surf board, people actually play in them when they are upwelling. so it could be theoretically possible to have something like that if is is tidally controlled with the perfect conditions. I'm still looking for a link about the island vortexes, I thought they were in Hawaii.

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Whirlpools are formed whenever two currents for a shear owning to differences in velocity. In a bathtub style whirlpool, the sheer is caused by the water accelerating down the hole faster than the surrounding water. This causes friction which is resolved by the generation of an ordered structure in the form of a whirlpool.

However, the most common whirlpools are those that occur in lateral flowing bodies of water. In many cases, they form under the surface so that are not observed in proportion to how common they are. When water is flowing around a bend, the water on the inside of the curve slows and the water near the outside accelerates. This will create a shear wall between the two that dissolve in to chaotic turbulences of lots of little whirls. The energy builds up the turbulence until the ordered whirlpool forms.

The whirlpool directs the water downward and by accelerating it creates a lubricating layer between the streams.

Such whirlpools can form and last for years. Some produce ice rings when the water on top freezes over.

To produce a sustain whirlpool that could suck down a ship. You would need either a single powerful current being bent along one side to create the shear or two current flowing suddenly adjacent.

In theory, you could get a whirlpool that forms like a tornado. Tornado's start when air get loft high by thermals cools and falls very fast downward and gets sucked but the thermal again. The forms a fast rotating cylinder of air with it's axis parallel to the ground. At some point, the cylinder goes vertical, concentrates and becomes a tornado. In theory, cold, dense high saline water crossing on top of warm, less dense water (optimally fresh) would sink rapidly, mix, warm and rise, drawing more cold into the system. If that cylinder turned on the side, you could have a sustained whirlpool.

In this case, you likely wouldn't have as sustained whirlpool but a dangerous region where such whirlpools could form without warning. Kind of an aquatic tornado alley.

If you want to suck down a ship with a plausible mechanism, I suggest aerated water. Gas bubbles in water lower it's density and ships become no longer buoyant. It's happened with fairly large vessels around volcanos and gas from hydrate beds is suspected in a couple of others.

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