I've just finished lighting up a cave I came across (while en route to see what this fortress business is about). Where the aforementioned hamlet is, just North West of that there's a blob of snow on the "other side of the river". Mooch around there and you'll find an entrance underground. A few waterfalls, lavafalls and iron ore / coal is all I came across, but there's some interesting rock formations down there.
I haven't mined anything down there, so help yourself.