Apologies if a thread along these lines has already been started - I did a search, but couldn't find a suitable match.
Whilst sites such as Göbekli Tepe or Stonehenge are obviously the result of ancient human ingenuity, planning and hard work, there are plenty of (almost certainly) natural structures around the world that, through erosion, volcanic activity, meteor impacts or various other natural processes including fossilisation, appear man-made and have fuelled countless tomes of pseudo-archaeology (yes, Graham Hancock, I'm looking at you!).
Thought it may be worth listing the top contenders here and using this thread as a catch-all for new candidates or fresh evidence emerging about some of the old favourites.
To get things started, here is a list of my top seven (in no particular order).
1) Yonaguni
Located some 26m below sea-level, off the south-west coast of Japan, this structure would last have been above the waves some 10,000 years ago, making it highly anomalous. Most experts, however, dismiss the step-like structures and apparent monoliths as the result of naturally occurring fractures along parallel bedding planes in the sandstone.
https://en.wikipedia.org/wiki/Yonaguni_Monument
2) The Baltic Sea anomaly
Some highly interpreted results of an indistinct sonar reading on the Baltic sea-bed have been compared to the Millennium Falcon.
Most experts however believe it is a natural rock outcrop.
https://en.wikipedia.org/wiki/Baltic_Sea_anomaly
3) The Richat Structure, aka Eye of the Sahara
A slightly elliptical dome some 40 km in width.
Its impressive symmetry have led some to speculate that it is artificial and it has even been cited as a location of Plato's Atlantis.
At over 2 million years old, it would be uniquely anomalous if artificial. The latest thinking is that it is simply the result of erosion of alternating hard and soft rock layers.
https://en.wikipedia.org/wiki/Richat_Structure
4) The Kaimanawa Wall
This structure on New Zealand's North Island looks not dissimilar to megalithic structures like Tiahuanaco and predates any (official) human occupation, but again has been dismissed as natural fracturing of the rock.
https://en.wikipedia.org/wiki/Kaimanawa_Range
5) Megalithic structures in the Ural Mountains
This is complicated by the presence of genuine megalithic tombs and carved monoliths in the region.
The claims made about unfeasibly vast rock walls being man-made though appear to be misidentified naturally fractured rock.
6) Bosnian and other "Pyramids"
Whilst roughly pyramidal in shape, these features in the area of Visoko in central Bosnia, have been identified as naturally occurring "flatirons".
I saw some very similar pyramidal hills on Madeira last year (see the Atlantis thread for a photo).
https://en.wikipedia.org/wiki/Bosnian_pyramid_claims
7) The Baigong Pipes
Curious tubular structures varying from 10 to 40 cm in diameter, found in caves in Qinghai Province, China.
Anomalously old evidence of irrigation or similar human infrastructure?
Or, more likely, fossilised tree roots or bamboo-like plants.
https://en.wikipedia.org/wiki/Baigong_pipes
Any more candidates for natural structures which can fool the credulous in to thinking they are artificial?
