Ponds without liners....

We live on a relatively small lot (50' x 200') and I was thinking about
putting a pond in the back yard in a particularly squishy area. The
soil here is clay based and if you dig down far enough (not too far at all)
it's just nasty clay.

Other than lined ponds, I can't say as I know much about "natural" ponds,
i.e. those without a liner. I'm wondering if I can make one by essentially
digging a hole deep enough. I suppose I could then either concrete it or
put in a liner of course, but the thought of setting it up au natural
intrigues me.

Anyone done a small scale pond without a liner or have any references?

James