One of the most astonishing structures is Yonaguni. A weird underwater ziggurat type of construction, about 30 meters beneath the water surface. Yonaguni is a Japanese island near Okinawa and Taiwan. In the mid-nineties the structure was discovered by diver Kihachiro Aratake in the clear but wild waters not far from the shore of Yonaguni.
It is a 5 layer platform hewn out of the bedrock, having straight planes and precisely defined sharp angles. It is about 200 meter wide and 30 meter high. Since it is 30 meters below sea level it is at least 10000 years old since in that time the sea level was - because of the melting of the icecaps at the end of the ice age - at a lower level than now.
This means that the structure is one of the oldest buildings in the world - together with the Sphinx in Egypt and Tihuanacu in Bolivia.
What its purpose was, is yet unknown.
Imagine that the structure is at the shore of ancient Japan, long ago. With its distinguished platforms, the weird shape and the enormous size, it must have impressed any visitor.
The site is difficult to investigate because of the strong ocean currents. Therefore still a lot of questions wait for underwater archaeology to be answered.

More information in Ref. H1