Soak
it with a water hose on slow drip for a few hours, let it soak
in for a few hours after turning off the slow drip so it's not mud
when you dig, and... use a pick axe. You may hit impenetrable layers of
caliche that you can only get through by brute force.
Not a geocaching question since we are not
allow to dig or use pointy objects to make holes but I can't see that that is
a problem for Arizonans in the desert areas since I find it virtually
impossible to dig a hole in this caliche
soil. I believe it's also known as hardpan. Just how to you guys
dig a hole, my gosh, I just wanted to plant a few cacti before the temps went
over 100F one morning.