One of the world's most iconic prehistoric monuments in the world is Stonehenge, which is in the Salisbury Plain of southern England. This monument consists of large standing stones set in a circle. Although no one knows for sure when Stonehenge was built, it is believed the stones were set in place in approximately 2,600 B.C. The two common beliefs on the monuments purpose are Stonehenge was either a holy site for Druids or an astronomical observatory.
Located in the southern England county of Wiltshire, the Avebury monument features three stone circles dating back to the Neolithic period, or the Late Stone Age -- approximately 2,600 B.C. These stones were buried for many years until the 1930s, when archaeologist Alexander Keller excavated them and returned them to their assumed original position. Avebury is open for the public to tour.
Sungbo's Eredo is an African prehistoric monument in the rain forest of southern Nigeria, near the village of Ijebu-Ode. Rice University's website states that Sungbo's Eredo may be the largest single monument in Africa. Although no one is certain who constructed it, it is believed Sungbo's Eredo was built as a rampart wall to defend a civilization. The wall stands approximately 65 feet high and runs for nearly 100 miles in the Nigerian rain forest.
Purbalingga is a regency -- or state -- in central Java, Malaysia, and in 2009 archaeologists unearthed 15 prehistoric sites. These sites are in near the Klawing and Tungtung Gunung Rivers. According to the Jakarta Post's website, some of the sites date back to the Neolithic Age, while others may originate from the Paleolithic Age -- over 6,000 years ago. As of 2011, the prehistoric sites at Purbalingga continue to be excavated.
The Casa Grande Ruins National Monument is in Coolidge, Arizona, approximately an hour south of Phoenix. This prehistoric site features the ruins of buildings constructed by the Hohokam Native American tribe. The ruins date back to the 1200s, when the Hohokam occupied the area. Casa Grande is on the National Register of Historic Places. A large ramada, or roofing structure, covers the Casa Grande ruins to protect them from inclement weather. Casa Grande was the first U.S. archaeological excavation site; excavation operations commenced in 1892.