Ponhpei is located just off the coast of the country of Micronesia. Researchers believe it could date back to the first or second century, but no records which could prove this exist.
There are 97 individual blocks, which are geometrically shaped, and separated by narrow channels of water - but the reason why is not entirely clear.
The city is 16,000 miles from Australia and 25,000 miles from Los Angeles.
Dr Karen Bellinger says: "As amazing as this site appears from satellite imagery, coming down to ground level is even more astounding. There are walls which are 25 feet tall and 17 feet thick."