This site, whose original purpose still eludes archaeologists, was unknown to European explorers until Father Kino, a Jesuit missionary, first recorded the site's existence in 1694. The area was set aside as federal land in 1892 and named a national monument in 1918. Although only a few prehistoric sites can be viewed, more than 60 are in the monument area, including the 35-foot-tall—that's four stories—Casa Grande (Big House). The tallest known Hohokam building, Casa Grande was built in the early 14th century and is believed by some to have been an ancient astronomical observatory or a center of government, religion, trade, or education. Allow an hour to explore the site, longer if park rangers are giving a talk or leading a tour. On your way out, cross the parking lot by the covered picnic grounds and climb the platform for a view of a ball court and two platform mounds, said to date from the 1100s.