Earlier quoted context omitted.
Unexplored by whom though? The Americas are a great example of places that might have made the list in greater numbers if history had gone a little differently, but those settlements were wiped out except for a handful of prominent ones. Then suddenly in the 1500s you get a lot of new Spanish settlements making the list. It doesn't qualify as continuously inhabited, but that doesn't make it unexplored. In many cases…
I probably should have used a different word. What I meant was that the last 200 years or so is the first time in history that human civilization has extended across the entire planet simultaneously.
I do think the concept of "continuously inhabited city" is interesting, but ultimately distorting in several ways, especially in the Americas. Adding the loaded word "civilization" just further muddies the waters.
However, if you consider that Antarctica was finally sighted in 1820, your figure of 200 years is very accurate.
That said, I think it is interesting that there are probably islands on which no human has ever set foot. At least I think that's the case, I don't have proof. Large swaths of ice as well, but islands are easier to think about.