Ore

"Illustration" courtesy of:  https://www.craiyon.com "piece of rock"

How many times has an explosion been "misidentified" as a "weapons test" above or below ground?  To be less specific, include those that were "mischaracterized" or "misrepresented".

None this century or last century.

With that being a "well-known fact", we can speculate about the source of a "suspicious" earthquake originating in North Korea "somewhere".

The "source" wasn't:

• A "misguided" attempt to carve an "underground city".
• An "open" try at digging a "swimming pool".
• The inevitable result of mining an "ore body" using something which should never be triggered near "ferrous" or magnetic minerals. 

Which isn't it?

2 out of 3 aren't unimaginable.  The remainder is horrific.

Visible evidence above ground may not be a depression.  It's living things that would bear a scare.  Do or don't bother looking from space - we aren't allowed to know or see such evidence.

Backward or "never think things through backward".