Fast thick pants

I'm not terribly given to reading poetry, although I suspect as I get older I might take it up. I have heard of Kubla Khan by Samuel Taylor Coleridge but never got around to reading it until last night. I was very impressed with these lines:

And from this chasm, with ceaseless turmoil seething,
As if this earth in fast thick pants were breathing,
A mighty fountain momently was forced:

Pants!