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A Google satellite view shows an unusual structure on a hillside in Miyazaki Prefecture, Japan (Latitude 31.782600°N, Longitude 131.233881°E):

Stepped structure on Japanese hillside

It measures about 170m by 120m and is structured in steps of around 10m, with some parts also showing a grid structure on a scale of around 2m. It appears to be made out of concrete. What could its purpose be?

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  • $\begingroup$ If you go into 3-D mode in Google Earth it's pretty obvious... they show up in a few places around there. Looks like they are built to protect canals and roads from eroding material on the hill during flash floods. $\endgroup$
    – f.thorpe ♦
    May 7, 2015 at 19:11
  • $\begingroup$ The view makes a lot more sense from the side, I'll give you that, though I don't see how it would help much in a flash flood way up on the side of a mountain. Is it supposed to catch debirs or is it the remains of debris that had been pro-actively removed? $\endgroup$
    – Volumetricsteve
    May 7, 2015 at 19:38
  • $\begingroup$ It's a means of stopping erosion of the hillside... just my interpretation... no source $\endgroup$
    – f.thorpe ♦
    May 7, 2015 at 23:23
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    $\begingroup$ It's really not that obvious. I think this question still needs a definitive answer: so far it's all conjecture, though @pont's answer seems plausible. Why there not everywhere? Who built it and when? Do they work? $\endgroup$
    – Matt Hall
    May 8, 2015 at 13:47
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    $\begingroup$ Re the 'off topic' flags — the question is about hazards (seismic perhaps?) and seems legit to me. Further there's precedent for unidentified manmade structures on sat photos. $\endgroup$
    – Matt Hall
    May 10, 2015 at 14:59

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The structure looks similar to this photograph of a "Japanese land retention system" mentioned in passing towards the bottom of this webpage. From the linked page:

Land retention systems in Japan, for example, are often designed as heavy waffle grids which are molded to the topography and cover it to a uniform structural depth.

This seems to correspond roughly to the structure in the question, although the "waffle-grid structure" is only evident in some parts. I've been unable to find any more details online about these land retention structures and their design.

As farrenthorpe writes in a comment, the three-dimensional view is probably more informative:

3D view

At first glance the siting seems strange, since there is no major settlement or road below the structure. However, it does overlook a river which, around 3 km downstream, feeds into the Tenjin water storage dam. In the current Google satellite imagery the riverbed is mostly dry and ongoing engineering work is visible in it, presumably also related to debris/sedimentation control. Similar hillside structures can also be seen on some of the slopes around the dam itself.

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