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In my writings I've created an island I need to hide. The island itself is around 100-200 square miles in area and the island rests around 12,000 feet in the air and is surrounded by a mountain range roughly the size of the Andes Mountains with a height of around 20,000 feet. The island itself lies in a valley about 800 kilometers at the longest, while resting around 200-300 kilometers wide. The island itself rests a minimum of 200 kilometers from the mountains on either side, and those same mountains mark the borders of two separate countries. The technological advancement of the world is similar to that of the 18th century, including the inventions of locomotives, basic automobiles, telephones and radar. The map below shows the two countries with the black line representing the border and the yellow area representing the location of the island. The blue shaded country is more technologically advanced, with elevated uses of radar and locomotive technologies. The red shaded area represents the magically enhanced area, with people having control over the four basic elements and other types of magic. Other than the discrepancies described above the world is nearly identical to Earth. My main question is would it be possible to hide such a city from the two countries, and what lengths would be necessary in order to accomplish this feat; whether through magical or technological means. as for surveilance of the two countries, as described above the technological country has access to most 18th century technologies. However the magically enhanced country relies on scouts and physical maps, and spells for detection of hidden items and/or persons is heavily limited. Continent

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  • $\begingroup$ How (and how high) is the city flying, and can you draw a map with relative positions and sizes if the city and the countries? $\endgroup$
    – Joachim
    May 5, 2023 at 23:47
  • $\begingroup$ Is "underground" an acceptable answer? If not, consider adding a bit more detail about what you'd consider "hidden", and what you'd still want the city to be able to do and how accessible and habitable it needs to be. $\endgroup$ May 6, 2023 at 7:54
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    $\begingroup$ You need to describe details of 1) the geography 2) the technological country's detection and intelligence systems, and 3) the magic country's detection and intelligence systems. If they have satellite imaging and/or the right spells, this is not possible. $\endgroup$
    – user86462
    May 6, 2023 at 9:53
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    $\begingroup$ Where's @WillK with his 'on a grain of rice' when you need him? $\endgroup$
    – user86462
    May 6, 2023 at 9:55
  • $\begingroup$ I think you confuse "island" with "city" (the city is suddenly being introduced, and I think the city is on the island), and "rests" with "lies". You also only mention the fact the island is flying in the title. I think this requires an additional edit. Pretend you're explaining your concept from scratch to a child, and everything should be clearer :D $\endgroup$
    – Joachim
    May 7, 2023 at 12:35

2 Answers 2

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  1. visual camouflage: from below the city look like only a large cloud.

  2. Radar camouflage:the city has a shield that absorbs sonar rays make it invisible to radar.

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  • $\begingroup$ Sonar or radar? (since sonar only works in water, probably the latter) $\endgroup$
    – Therac
    May 6, 2023 at 0:18
  • $\begingroup$ Sonar works fine in air. Bats use it. $\endgroup$
    – Mary
    May 6, 2023 at 0:31
  • $\begingroup$ Sonar works in air - the difference is that Sound travels at ~330m/s in air, whereas Radar (an electromagnetic wave) travels at the speed of light. Water absorbs Electromagnetic waves, so Radar isn't possible underwater, but the speed of sound in water is higher at around 1,500 m/s - so Sonar is both practical in Water but also more effective than in air. $\endgroup$ May 6, 2023 at 3:45
  • $\begingroup$ Does the sonar shield absorb magic? Not having a go at your answer, but the vagueness of the question. $\endgroup$
    – user86462
    May 6, 2023 at 9:54
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    $\begingroup$ @AncientGiantPottedPlant as you said the question is vague. So I can't say. $\endgroup$ May 6, 2023 at 22:46
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The trivial answer is: make it fly high.

But even if it was at the bringe of space (100 km), it could still be detected by your two countrys without much effort. Even small satellites of 7 meters length flying at 550 km are a pain for the astronomers. So, detecting a whole city woud not be a big deal - even by making it flying very high.

The second trivial answer would be to use a cloud to hide your city. While this worked in Ghibli's Castle in the Sky, you specified that your city had to remain still - and a huge cloud standing still in the sky would raise suspicion very quickly.

So, you will need to use more advanced ways.

You make it invisible

It is theoretically possible to make something invisible by bending the light around it. infact, this has been already achieved for infrared wavelengthes with plasmonic metasurfaces.

The phenomenon is similar to the distortion of the path of light in a mirage or over a fire, but much more refined. In your world, you could make it work with simple magic creating onion-like layers of hot air around the city.

The problem is, that the magical faction could already know this enchantment and have some clues due to the weird winds that warming the air around a huge structure would cause. Also, the tech faction would find some of its airships flying over the valley not coming back - because the planes crashed on the city hidden on the middle.

You make it transiting between dimensions

According to the string theory, our universe may have more than just the four base dimensions. If you were able to make the city migrate in another dimension, it would still be located in the middle of the valley, but in another plane of reality. If the ground factions don't have the ability to transit between dimensions, your city could remain hidden for many centuries.

However, it may still be possible to detect the city with advanced detection system. This becomes highly speculative, but for example you could still feel the gravitational trace of the city. You could even explore and map the city's ghost trace with a high-precision gravimeter, so maybe the tech faction would be better suited to actually detect the city.

On the other hand, transiting across dimensions - and live to tell it - would be an incredibly difficult task even with advanced techs. Accessing the city might hence be easiest for the magic faction.

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