They had initiated a survey by 1598 (when King Philippe II/III ascended the throne) which was followed up by an even more intense survey effort once the Treaty of London 1604 was promulgated.
Early ~ 1500s - settlements had a variety of problems with a rather severe drought on the East Coast (that may also have affected much of the Midwest and Great Plains) .
This field is wideopen for internet archaeology. There are all sorts of private records out there that make mention of things that could only be known by someone here before the subdivision of North America. Some of them are backed up by archaeology ~ many aren't ~ but there's plenty of opportunity for the astute observer to make some real finds.
There are no easy answers ~ like "the Vikings did it" ~ most all of the Viking finds involve little more than the use of a type of Rune alphabet well known by 1500s Swedes. Sure, it'd be great to find some Vikings, but who are those Swedes here in the 1500s? That's equally enigmatic.
A mystery I'm leaving for others is the old town core of Helena Mt. There are two of them! One North of the crick. One South. The crick was full of gold. Both town layouts conform to the rules and standards of the Spanish "Law of the Indies" ~ these things were old when the locals turned into the Comanches and rode off to raid in Texas!
Local amateur archaeologists have probably got an idea of whether or not this is a pre-Jamestown settlement ~ accessible from the Columbia river, but they haven't published, so they must have found some truly interesting stuff that will allow them to write books that others will pay good money for.
I'm sure there's something to look for there but has it been found. Time will tell.
Cathedral yields more surprises: Crews unearth Presidio chapel remnants
Blessed Junípero Serra 1713 - 1784 (Mission Chronology, Biography, etc.)
The Significance Of Blessed Junipero Serra (pictures of Missions)
Start a Serra Club in your area and support vocations to the priesthood and religious life.
A Norse settlement from about the year 1000 has been excavated near the northern tip of Newfoundland, at L’Anse aux Meadows. The items found there are definitely of European origin, not Native American. Of course the settlement doesn’t seem to have lasted very long. It isn’t clear if any of the Norse explorations reached the areas that are now part of the US. There was a medieval Norwegian coin found in an archaeological site in Maine, but perhspa it was traded from one tribe to another, so perhaps was acquired through contacts in what is now Canada.
the “Swedes” there in the early 1500’s were probably Norwegian fishermen. Norwegian and Basque fishermen frequently fished the Grand Banks before Colombus.
And the “proof” of the Vikings settlement is actually a spindlewhorl found in ancient housing. Local Amerindians didn’t use spindlewhorls.