How do you waterproof a pond?

lay 6 mil plastic down, or dump bags of clay in and spread it ,

To waterproof a pond you must create some way to keep the water inside your hole. To continue what was suggested above, you can use clay that will expand when exposed to water and form a somewhat waterproof barrier or you can line your hole with some kind of a material to act as a barrier.

Another of many liner options is to use EDPM. This is a flexible rubber that you place in the hole. The advantage here is that it will stand up against weather and UV exposure. It is guaranteed for 20 years if you don't plan to have a pond for longer than that.

I have used 6 mil plastic on temporary ponds. It is very thin and does not stand up to abrasion or UV exposure so it is not suitable for long term use.

Before you begin your project I would suggest that you do alot of research. The time you spend planning will reward you with many fewer issues after construction is completed.

Take a look at http://www.humboldt-county-lines.com/index.php?option=com_content&task=view&id=183&Itemid=58 as a starting point for some of the things to think about. (www.rosepond.com)