In addition to what Bill C said...there is a type of pond called a water table pond (I have one). In my pond the "soil" is mostly gravel, sand and everything else that won't seal. My pond works because the water table is very high, maybe 2 feet below the surface most of the year. There are times when the water table drops lower though. If your test holes show poor soil, don't give up immediately until you have watched the test holes for a year. If your soil is not a good clay that will seal, yet the water table stays within 5 feet or so of the surface, a pond might still be possible but you will need to dig it much deeper.

Please keep in mind, I am not a pro, just giving my personal experience

[Linked Image]
Be Brave Enough to Suck at Something New!