Pond Boss Forums POND CONSTRUCTION Selecting a site Test hole

I'm no more expert than DIED, but here's my $0.02:

I would dig down as far as the bottom of the pond, or deeper if it was needed to find suitable clay for sealing. You're looking for good soils (clay) to use to hold water and bad soils (sand, gravel, organic layers like peat, bedrock) which would be prone to leaking, with the hope of lots of good and not too much bad. You should be getting the idea on how deep you will have to go to remove topsoil or other less desirable material on top, and if there exists sufficient quantities of clay for sealing.

The test hole doesn't need to hold water itself, for the reasons DIED gave, but should show you if a pond can be constructed with proper techniques using the materials on hand which WILL hold water. Or you may find the site has very unsuitable soils, and be out the cost of the test holes instead of a major pond excavation effort.