^ Then it's not permafrost. Significant permafrost coverage doesn't extend any further south than somewhere around 55 degrees north in North America, aside from in mountainous regions. That's just regular ol' frozen ground that caused the flooding.
Well, it was called permafrost when I learned about it in elementary school, so I don't know what to tell you.
Huh. Your teachers must have been rather misinformed...

The reason it's called permafrost is because it is, quite literally, permanent frost. In other words, the ground remains frozen, even during the middle of summer, and doesn't undergo a seasonal freeze and thaw. Which is why you generally only find it in the far north; it gets far too hot down here during the summer for the ground to remain below 0C. So the reason I was surprised at the basement comment is because most buildings built on permafrost are built on stilts, to prevent the heat from the building melting the permafrost, and the building collapsing.
Anyway, apologies for the information overload, but my degree is in geography, so we covered permafrost pretty extensively in some of my classes.