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(This is part of a game I'm making, with a bit of backstory here.)

Details:

Size/Shape: Roughly oblong and quite large. Length of 15 miles, and a greatest width of 8 miles.

Placement: 500 miles south of Perth, Australia.

Environmental effect: Due to handwavium reasons, the appearance wouldn't significantly effect the surrounding environment. It can be seen, landed on, all that. However, it won't effect sea levels or anything. It will be sensed by all modern technology and biological functions (of humans AND animals).

Land: It will be covered in a temperate rainforest, in case that helps anything.

Timeline: The appearance of the island takes 6 hours. This finished taking place right "now."

Method of appearance: A thick fog rolls out of the ocean, and over the course of 6 hours, the topmost edges of it seem to solidify into a real island. From below it looks like fog that you can't see through. From the top, like any other island.


So, how and when would an island this size, in that position be discovered?

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    $\begingroup$ @Mazura appreciate the thought. I can see this is a sticking point, so I will give away this plot point. The island is functionally (to the environment) the same as a dense fog. To the senses of humans, animals, and current sensors (on sats and whatnot), it is a real island. It sit atop the ocean, without displacing water. There would be no seismological events. This is important to the story and can't change. LATER there is a part where it becomes "too real" and has the problems you and stephenG point out, but that is 20 years after it first appears. Again, I really appreciate the comment! $\endgroup$ – Aethenosity May 26 '18 at 17:41
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Days, if not hours

200 nautical miles is the exclusive maritime territory, which translates to about 370 km. Any Navy ship patrolling that region will spot it at the edge of its range and investigate.
EDIT: I would expect a routine patrol would pick up a large unidentified object at extreme range and call it in. Wouldn't do to let an invading armada pass by, just because they had to go out of their normal patrol route. HQ will probably inform any satellites passing by to take a closer look. If satellites aren't available within a few hours, they'd send a couple of jets to fly by at high speed. "Island? What do you mean, just an island? There's no island there!"

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    $\begingroup$ Are you sure they would investigate, and not just assume everyone already knows about the island? $\endgroup$ – John Dvorak May 26 '18 at 11:45
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    $\begingroup$ Unless there's some kind of memory-charm effect you didn't mention, they'll know that they didn't know about the island. In case of any kind of confusion, they'll presumably refer to their charts, which also won't have it. I assume they'll then check the charts against somebody else's (which also won't have it)... at some point they'll reach the right conclusion (that it's new, because the alternative is every nautical chart in Australia being wrong), and then they pretty much have to investigate it. $\endgroup$ – Cadence May 26 '18 at 12:08
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    $\begingroup$ It occurs to me that, depending on the exact range and means of detecting the island, they might conclude that their charts are accurate and it's the detectors that are in error. That might put off discovery for awhile, depending on their reaction. $\endgroup$ – Cadence May 26 '18 at 12:19
  • $\begingroup$ You forgot Google Maps $\endgroup$ – pojo-guy May 26 '18 at 12:51
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At first I was agreeing with nzaman (who just got my upvote) but then I hesitated because of the explanation and decided to write my own answer.

  • The area will be overflown by military recon sats several times a day. Those pictures may or may not be examined humans quickly. I wonder if there are computer algorithms that would detect such an unexpected event.
  • The island is large enough to show up on civilian and military weather sats and those pictures will be examined in near realtime. The question would be just how closely the images are examined -- the discoloration might be mistaken for a cloud at first.
  • The appearance may cause seismic events, depending on the mechanism. Does your handwaving cover that?
  • Maritime patrol aircraft don't provide 24/7 coverage that far out, I believe. The Fleet Air Arm has no more fixed-wing craft. The RAAF has a total of 26 Poseidons and Orions, based in RAAF Edinburgh but I guess they are oriented northwards ...
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    $\begingroup$ @Vorac: It's not that it's not possible for software to detect a new island in satellite imagery, it's just a question of whether anyone has bothered to create automated checks for new islands appearing in satellite imagery. I would imagine not, because nobody expects that to happen. And if software isn't flagging the imagery for a person to look at, probably no one will (there's too much for humans to be looking at all of it.) $\endgroup$ – kundor May 26 '18 at 17:38
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    $\begingroup$ Regarding satellites: 500 miles south of Perth is roughly 40 degrees South latitude, in a region that is covered by clouds 80+% of the time, which can seriously impede satellite discovery. $\endgroup$ – Eric Towers May 26 '18 at 18:20
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    $\begingroup$ @Vorac: It could even be worse than that. I could very well imagine that if software is used to analyze the difference between two areas, there could very well be a heuristic to waive any change bigger than NxM; a simple sanity filter to avoid a big cloud/cloud shadow triggering too many false positives... $\endgroup$ – Matthieu M. May 27 '18 at 14:05
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    $\begingroup$ @MatthieuM. so no one would care if a country deforested 50sq km overnight for a military airfield? We are all just guessing here. $\endgroup$ – Vorac May 27 '18 at 16:45
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    $\begingroup$ @Vorac: "Guessing"? Not at all! We're World Building. This is the kind of "flaw" that is likely enough for an author to explain some thing :D (I realized now that I did not mention that I was thinking exclusively of maritime areas, because no object so big could appear in a maritime area obviously). $\endgroup$ – Matthieu M. May 27 '18 at 17:42
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It could take a very long time

Sandy Island is a phantom island that was added to maps in 1876. There are also mentions of a "Sandy I." as early as 1774, but it's unclear if it was the same island. It was about 1,000 km north-east of Brisbane.

It was removed in 1974 by the French Hydrographic Service and in 1985 by the Australian Hydrographic Service after flybys, but other countries kept it in their maps. It ended up being visible on Google Maps until 2012, when a surveyor ship actually went there and found nothing.

Now, we do have lots of satellite imagery being taken every day, mostly weather and environmental stuff, but also mapping. However, unless somebody is actually continuously running image comparisons on all of them, they are useless for finding new islands. Especially one that's 300 km2.

I would guess that there are some very smart image comparison algorithms out there running on very localized, high-value targets around the world, but terrorism and fuel enrichment plants are rare in the south-eastern parts of the Indian Ocean.

This is a screenshot from MarineTraffic right now where I added an orange dot 500 miles south of Perth:

enter image description here

The shipping traffic in the area seems negligible. Since you seem to have explicitly disallowed ways other than visual to spot the island (such as seismic events), unless you put the island smack in the middle of a busy lane and have ships bump into it, you could stay undetected for a long time.

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How: most likely by commercial shipping traffic departing from South Africa or Fremantle to New Zealand or South America.

When: probably within three weeks to a couple of months. This section of ocean is very lightly trafficked and patrolled.

https://goo.gl/images/XycHt7

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Three business days. That would be the most likely time, 50% chance it's sooner and 50% later. Takes two days to get the first images, one business day to process the discovery.

There are ~150 observation satellites orbiting Earth, with the typical orbit being 800km. Each satellite scans about 200*40,000=8,000 km^2 every 100 minutes. Earth's surface being scanned is ~360 million km^2 - the other 150 million near the poles is largely ignored.

This amounts to 120,000 km^2 or 1/3,000 of the surface scanned per minute, or 1/50 per hour. There is a lot of overlap between scan areas, hence the 50% qualifier in the beginning of the answer - can be sooner, can be later. Still, in four days, it's all but impossible for the island to stay undiscovered.

While earlier discovery by shipping or aircraft is possible, it's highly likely to go unreported due to simple assumption one is mistaken and the "someone else's business" effect.

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    $\begingroup$ Regarding satellites: 500 miles south of Perth is roughly 40 degrees South latitude, in a region that is covered by clouds 80+% of the time, which can seriously impede satellite discovery. $\endgroup$ – Eric Towers May 26 '18 at 18:21
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    $\begingroup$ @EricTowers That's some very beneficial info for my story, I hadn't considered that. Thank you! $\endgroup$ – Aethenosity May 26 '18 at 19:35

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