These black or brown deposits are caused by iron (with a bit of lime).
It is not dangerous, although it looks a bit disgusting, the (metallic) iron is only ingested in very small quantities.
It is difficult to remove. The lime can be removed with a lime remover (available at chemists), the iron with an acid (for example with lemon juice or cleaning vinegar).
This does require some patience.
Alternate between a night in the limescale remover and a night in the vinegar.
Gradually it will become cleaner. You probably won't get it completely clean, because the plastic of the kettle will eventually discolour because of the iron.
If this is tap water, you should consult your water supplier, because it could also be the pipes of your own system.
Have you checked with your neighbours? Also try to make hot water with bottled water for a while.
Then you can draw your conclusions and take the necessary action. If it comes from iron in the water, it should be slightly brownish.
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