This was a mind-bogglingly huge hole in the ground. (Well, mind-boggling given their technology, anyway.) It's a little hard to make out in this photo, but this is a huge, round hole, about 9 feet deep and maybe 80 or 100 feet across and lined with carefully cut stone. There are sloping paths that lead down into it, for individuals to go and collect water. And it's not longer visible to the naked eye, but the archaeologists tell us that there's an aqueduct leading out of it and down the mesa to agricultural terraces. It holds a huge amount of water (in some units that I don't recall). It dates to roughly 1000 AD.

An interesting cultural note: there was a plaque near the reservoir that said that in 2003, the American Society for Civil Engineering recognized this site as a "civil engineering heritage site" (or something along those lines). Nifty...