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In a setting with a planet have similar atmosphere as earth with higher ocean coverage, I need a good reason for human on with equivalent of 16-17th century technology to avoid navigate through deep ocean, yet it's easy to navigate in shallow waters.

For example, It should be technically easy for most regular merchants to sail from Spain to Egypt, but from Spain to Cuba following Columbus' route should be impossible and suicidal even using the most advanced technology human have and ignoring cost. In this case I will prefer to have the depth/altitude of water being the main indicator.

Several possible way I ruled out for plot reason:

  • Magic

  • Continuous modern/future technology: I am not against higher technology as the root of cause, but such element shouldn't be a continuous effort. For example:

    Good: Some nuclear war 5000 years ago left some fallout

    Bad: Some Alien spaceship will shoot and sink any ship passing

    And of course, I need it to be (somehow) make physical sense

  • Lack of destination: It's not a specific place that nobody ever been, it should be a general effect like even you know your destination is on the other side of ocean, yet you still have to do the long route in shallow water. Navigation map system should be pretty completed given the technology human has during 16-17th century.

And again just to highlight, it shouldn't be a cost/effect or risk/effect issue. Hopefully I want to create a general belief among most human that

Entering deep ocean = Death

Some visible/detectable effect that can enhance the fear is a bonus.

Edit:

I have read through the question and answers here, and in this case I am not interested in making a particular area of ocean not navigable, instead I need some good reason to make the phenomenon in whole planet level, and such effect had better to be related to altitude.

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marked as duplicate by Rob Watts, sphennings, Separatrix, Frostfyre, MolbOrg Jun 14 '17 at 17:19

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ have the phenomena on whole planet $\endgroup$ – MolbOrg Jun 14 '17 at 17:27
  • $\begingroup$ @MolbOrg case is that when certain phenomena been widespread in planet level (for example, wind), it may come to a point that you can't explain it by anything other that "Magic" anymore. That's why I believe this is a different question, since the origin questions specifically mentioned "certain large bodies of water be impassable". I so far only find the shipworm answer as something can get spread naturally in planet level, but I hope to grab more ideas in this question when the prequel is different. $\endgroup$ – tweray Jun 14 '17 at 17:45
  • $\begingroup$ The trade winds are very much a thing and only allow certain courses to be sailed across the oceans by the ships of the era. They create right and wrong ways to sail round the world even now. If those winds and currents were more adverse it could easily set the first crossings of the great oceans back another century or two. $\endgroup$ – Separatrix Jun 14 '17 at 18:12
  • $\begingroup$ @Separatrix yes, and that's why it require quite an effort to make it meet my requirement while keep it away from magic. Since what I hope is a "dead zone" wide spread among majority of the planet constantly, instead of things like I can't get B from A but can get A from B. Or something like I can only get B from A between July and October. And obviously "whenever you get on ocean the wind is always against you" doesn't seems scientific. $\endgroup$ – tweray Jun 14 '17 at 18:25
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    $\begingroup$ @tweray, it's simpler than that, if the wind is too strong it kills you, if the wind is against the current it kills you, if only the current is against you your ship isn't fast enough to sail against it. Sailing in the deep Southern Ocean either kills you directly or you get hopelessly lost. Sailing round either Cape kills you and if you can only go one way round the world, getting from Europe to America and back requires you to go round both Capes. Sailing in the open ocean is dangerous, it always has been and it remains so, all you need to do is turn it up a notch. $\endgroup$ – Separatrix Jun 14 '17 at 18:30
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Simplest thing I can think of is the existence of large and dangerous ocean creatures, large horned whales and the like. Perhaps they are attracted to the movement of the ships on the surface and track them once they enter the deeper waters where they reside.

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