Hi, Dr. Bill--no, I don't even know what an astrobleme is.

And stales, perhaps you can help me here, or you, CK--I'd had the idea that a chimney (vertical, narrow, yup) was something that was formed naturally--usually in rock. But I thought the fact that it was dug would qualify the opening down to the plane as a shaft. Otherwise, why don't we say, "going down the mine chimney"?