My two cents about mars "holes":
http://lc84.altervista.org/marte/buchi%20marte.html
It's in Italian, but maybe online translators can help you.
In a few words: no holes, "just" lakes. WATER lakes (also
water appears BLACK from space, being it reflecting SPACE rather than SKY).
meteor crater + ice + volcanic region = water lake
i.e., ice in a meteor crater was melt by underground lava thus creating the lake.
About low aire pressure and water vapour... don't know, maybe ther's actually a water spring in the crater? It would freeze under normal circumstances, but it wouldn't if warmed up by underground lava!
But I don't think ALL "seven sisters" are lakes: most of them are just deep craters; but I am not able to download high resolution images of them: can anybody help? Can anybody download hi-res images of 7-sisters and post crops of the interesing parts?
Thanks.