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I need to create a world with some very high plateaus and some very deep depressions. The sides of these structures should be very steep and unclimbable. They need to provide an obstacle sufficient to ensure that the the plateaus and depressions are only accessible from the surface by air transport.

The heights and depths also need to be sufficient to ensure that the climates are very different from the surface, but most areas should still be habitable when reached, ideally 3-6 miles above/below the mean surface elevation. The plateaus should be of the order of 100 miles across; the depressions (craters?) should be of the order of 1000 miles across.

Assume an approximately earth like world except with much less water. The depressions must not simply be oceans, although some water may be present. Other than that any physical characteristics can be adjusted if this helps. If this is not possible what comes closest? No magic.

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  • $\begingroup$ Grand Canyon? I am not certain that what you describe has been observed in our solar system, and from that perspective it seems a natural explanation may be hard to find. Perhaps a cataclysmic even like a large asteroid strike could form an enormous feature on the scale you describe, but it would erode and not meet your criteria. Perhaps the features are constructed by ancient races. I wonder if you could solve some problems by introducing other elements - The walls are unscalable due to a type of hypothetical (toxic) plant that grows on vertical surfaces, for example, rather than geometry. $\endgroup$
    – SFWriter
    Commented Sep 16, 2017 at 15:03

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You want tepuis.

tepui

Tepuis are the remains of a large sandstone plateau that once covered the granite basement complex between the north border of the Amazon Basin and the Orinoco, between the Atlantic coast and the Rio Negro, during the Precambrian period. Over millions of years, the plateaus were eroded and all that were left were isolated flat-headed tepuis. Although the tepuis looks quite barren, the summit is teeming with life.

Tepuis are between 1000 - 3000 meters high; 3000 is 1.8 miles so close to what you want. The tops of many of these plateaus are inaccessible from the ground; you need a helicopter. Or a house floated with many helium balloons.

Auyán-tepui

is a tepui in Bolívar state, Venezuela.1 It is the most visited and one of the largest (but not the highest) tepuis in the Guiana Highlands, with a summit area of 666.9 km2 (257.5 sq mi) and an estimated slope area of 715 km2 (276 sq mi).

enter image description here Auyan-tepui image from http://c8.alamy.com/comp/A415PF/auyan-tepui-guiana-highlands-venezuela-world-heritage-clouds-precipitous-A415PF.jpg

Tepuis are real, super cool, and you need exaggerate them only slightly to achieve what you want for your world.

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  • $\begingroup$ Yes good try, but it needs to be a lot higher. According to the link you provided Jimmie Angel, a bush pilot searching for gold ore in 1937 discovered Auyán-tepui in 1933 and attempted to land there in 1937, but crash landed and had to “hike down the sloping backside” which took 11 days. $\endgroup$
    – Slarty
    Commented Sep 17, 2017 at 8:11
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    $\begingroup$ They are not all straight up. Some collapse. But in your world you can make none collapse and no way down anywhere, if you like. The highest right now is 1.8 miles. But in your world you can make them higher. Make them older and the sandstone bed they come from deeper. The principle is sound and you can adapt it for your fiction. $\endgroup$
    – Willk
    Commented Sep 17, 2017 at 14:16
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This is entirely possible. The depressions will be colder, of course, and the high summits will be much hotter. Clothing and limited technology can make them both habitable. You don't need magic, just some great descriptions since you have a very specific landscape in mind.

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