Diquís Spheres
The Diquís River Delta in Costa Rica is dotted with about 300 of these large, sculpted spheres. The largest of these stones is over 2 meters in diameter, making it larger than most average humans, with a mass of nearly 15000 kilograms.
The spheres are made mostly of local igneous gabbro, with an occasional sphere of other local materials such as limestone. Since the rocks are much older than the carving, geochemical techniques struggle to date the time that they were carved. Instead, scientists estimate that they were carved between 600 and 1400 AD based on the layers of sediments that surround some of the boulders.
The original reason for their carving is unknown. When they were first recognized as artifacts in the 20th century, several of them were destroyed or even dynamited due to rumors that they may hold gold. These boulders and other carved rocks in the region were added to the UNESCO World Heritage list in 2014.
-JBB
Image credit: http://bit.ly/29aYEfQ
References: http://whc.unesco.org/en/news/1160 http://news.bbc.co.uk/2/hi/science/nature/8593717.stm http://bit.ly/29a5aT3 http://bit.ly/29ad43s http://whc.unesco.org/en/list/1453