nan Madol in the middle of nowhere, even today it'd be extremely hard to build such structures and the myths surrounding the construction are just crazy.
They have ruins in Antilla, Turkey, which are also close to the shore. It was made from materials that date from millions of years ago and yet, they find metal rods similar to rebar in the construction. BTW, the material is much better than our modern steel, it has zinc in the composition, which have allowed the metal rods to last until today