What is the most mysterious monument in the world that nobody has managed to explain yet?

Dighton Rock was originally located in the Taunton River in Berkley, Massachusetts. It was partly covered at high tide and has since been moved to a museum. The rock is covered with petroglyphs, the origins of which remain unresolved. The rock is 5 feet high 9–1/2 feet wide and 11 feet long. It originally faced the water. The petoglyphs are badly worn and are difficult to make out.

Photograph by Frank S. Davis, September 11. The author stated: " One of them is dated September 11, 1893; the other two were taken the winter following. I do not remember to have copied any previous photographs or drawings in making my markings on the rock. Going back to my school days, I can remember going to the rock and taking chalk and marking in the lines. There would be a number of us and we would all work at it and talk about what they were put there for. So you see I had seen the lines marked in a good many times. Then in after years I got a camera and got quite interested in taking pictures. One of the photographs I made for some one who was writing a book at that time, but cannot remember the name of the writer or that of the book. I expect that I marked quite a few lines on the rock which were never put on by the maker; also that there were quite a few marks put on by the maker which I did not mark in. The rock was so worn away that it was very hard to trace the markings, and not knowing what the figures were one had to use his own ideas in connecting the markings. You will notice the one taken in September is not as complete as the one taken later; possibly it is nearer right than the one taken the following winter which I tried to fill in more. The marks low down on the photo I do not think amount to much."

“An exact copy of all symbols or petroclyphes by the Historical Commission of Providence, Rhode Island, published 1830.”

Kenneth C. Zirkel / CC BY-SA (Creative Commons - Attribution-ShareAlike 4.0 International - CC BY-SA 4.0)

All the photos come from the Wikipedia entry on Dighton Rock.

Most likely, in my opinion, the petroglyphs were inscribed by early indigenous people in the area, but probably predating the known tribes that were here before European settlers arrived.

Other theories include the Portuguese explorer Miguel Corte-Real who reached Labrador and disappeared in 1502. Norse explorers, ancient Phoenicians and even Chinese explorers.

Unfortunately the petroglyphs are so worn it is difficult to tell their original shapes so they may always be a mystery.