The second case is actually, if anything, a little bit more concrete. It is essentially the existence of the Colombian Muisca tribe, which has been said to be responsible for the origins of the El Dorado legend.
There are different accounts, but the basic idea is that a people called the Muisca (whose descendants still live today) occupied Columbia thousands of years ago, and established a tradition of burying old kings and crowning new ones via a great deal of ritual.
That ritual included coating people in gold dust and stacking rafts with rich golden artifacts to be sent into a special lake.
It is largely accepted these days that Muisca traditions were distorted in tales, ultimately leading to tales of an entire city of gold.
This seems a satisfying explanation for the hyper-rational, but at the same time, it seems almost difficult to believe such a straightforward set of rituals being so dramatically misinterpreted.
This begs the question: might the ancient Muisca people have established something more closely resembling a small city of gold? Might it in fact now be at the bottom of the aforementioned lake?