So no "modern" man built any buildings until the 20th century. What I suggested was that these were "ancient" building foundations. I have included the link below, hoping Tomnod will still let us access the map tile(s) since this campaign is over. I know it is in their archive.
If you look closely at the shape of the "rooms" you'll see they are not the form we use in these "modern" times. They also appear to be made from the same type of rock, or at least the same color.
Chinese, Egyptian and Phoenician artifacts have been found in northern Australia, so we know that in ancient times global seafaring/trade was done. Ancient maps showed the coastline of Antarctica and mountain ranges - all ice-free - something "modern" man couldn't do until the advent of ground-penetrating radar. One mountain range - under nearly 3 miles of ice - wasn't discovered until then, but it was shown on the ancient maps. Makes one stop and think just how much we don't know and how much we're being kept in the dark about.