You can make a perfectly good drill bit from a piece of PVC with teeth or serrations cut into the end.
These teeth dig into the dirt (or clay or sand) and grind it up into small
cuttings that can be washed to the surface. The bit can be cut with a
hacksaw or a file. I like to do it with a bench grinder as it is quick and
easy. Below is a picture of one cut from a two inch piece of PVC with a
bench grinder. Click on the video to play it.