In the spring of 2020, there was news that during a strong low tide a strange stone spit appeared on the Sakhalin coast - a road going into the sea:
Found by local residents at low tide. Someone even decided to drive a few meters along it. Looks like a scythe or dock for small ships or boats. Nothing out of the ordinary from afar until you start looking at the structure of the stone:
This object looks like a polygon masonry. And since the "road" is not flat, but rounded, an analogy to a tunnel or a large pipe suggests itself.
You can see in detail the breed of this "road" or braid in the video:
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Whether this structure is artificial - no confirmation has been received. Therefore, only versions can be made. That this is either an antediluvian marina, or just a natural formation. There are quite a few similar examples in favor of the second option:
Promotional video:
Coast near Murmansk.
about. Shikotan, Kuril Islands.
White pocket on the Paria plateau in the USA.
In geology, many such formations are called traps. They are supposedly of volcanic origin. But look at this photo:
Vermillion National Monument.
Very similar to the fossilized and cracked outcrops of a mud volcano. Cracking like polygonal masonry. It is also clear that these masses expanded in volume like yeast dough. This is possible in the presence of bentonite clays in the liquid rock.
My opinion is that once, in a turbulent geological time, similar masses flowed from the coast into the sea, forming "tongues" and stone pipes, turned to stone and cracked.