Hole-in-the-Rock

In Phoenix's Papago Park, there is a small hill made of red sandstone with large cavities, called tafoni, caused by erosion. One is so large that it creates an opening to the other side of the rock that can be hiked up to reach great views of the park. 

The rock features in what is now Papago Park are estimated to be around 20 million years old. Millions of years of erosion created the openings in the sandstone for which this landmark is aptly named. Archaeologists believe that the formation was used by the Indigenous Hohokam people as a sort of calendar. There is evidence that markings in the rock were made to indicate the position of the sun shining through the opening in the rear side of the rock, which differed throughout the seasons.

The rock formation is reachable either by hiking up a flatter, easier path behind the rock, or a steeper climb straight up the front side. Note that the latter choice is not recommended for inexperienced climbers.