Wikipedia talks about several volcanic craters on the island that are no longer active, i.e. anciens cratères in the sense of former volcanic craters.  Perhaps this is what was meant.

Bonjour Adrian,

C'est une bonne question ! I instinctively wrote "ancien cratère volcanique" and got this one right. You are correct in that when "ancien" is before the noun that it modifies, it usually means, "former" or "ex", (as in boyfriend/wife/teacher, etc). In this case I chose to put it before the noun because of the use of multiple adjectives: "ancien" and "volcanique".

I seem to remember a lesson on this exact subject - the use and order of multiple adjectives, but it is not linked to this exercise. I read over the lesson on "Ancien- before and after the noun" and even watched the little video, but there is no mention of a case like this. 

Maybe someone else can offer a better explanation ?

Merci et Bonne Continuation.

Frank: that sounds plausible. I checked, and Mafate is indeed extinct. Would be great to get confirmation from the author!

Hillary: It doesn't appear to be in any of these: https://progress.lawlessfrench.com/my-languages/french/notebook/866848

That is a tricky one. Happy with Frank's answer, as it is apparently both an ancient and a former volcano. However I wonder if you would also be able to write "un cratère ancien volcanique", or "un cratère volcanique ancien" as well as "un ancien cratière volcanique" ? These answers are not accepted in the exercise but it is usually possible to have more than 1 adjective after the noun.