We started from First Water trail head as though we would do the loop but at the saddle about halfway around we took a short side trip into Jabez Clapp's 250' long tunnel into Hackberry Mesa which some refer to as Hackberry Mine, although was supposedly called Frog Peak Mine some 50+ years ago.
We found nothing but five mice (four alive/one desiccated) and two bats in the tunnel. I say "tunnel" rather than "mine" because it does not appear to have any geological significance that anything valuable was mined here. However, it is immediately adjacent to a huge depression that may have been a cave-in, so maybe the original mine caved in and this tunnel was an effort to dig in 200' from the left side, then turning toward the right, ending at 250'.
Although some photos didn't turn out the best, here are the photos I took on the hike:http://www.changephoenix.com/10/2010-09-22HackberryMine.html
Spending more time than expected at/in the tunnel, we took a short side trip up west from the saddle for some photos from above Hackberry Spring, then returned along a trail more or less straight back to the old windmill/corral area then up and over onto the road and back to the trail head.