Adam’s Calendar is controversially suggested to be the oldest man-made structure in the world.  Sometimes referred to as 'African Stonehenge', it predates both Stonehenge and the Great Pyramid of Giza by tens of thousands of years. Located in Mpumalanga, South Africa it is a standing stone circle about 30 meters in diameter. 

It had been discovered by in 2003 by the pilot Johanne Heine, who had been photographing other ancient stone circles in south Africa for the past 20 years. During one of their flights, a fellow pilot of Heine ended up crashing his plane and during the rescue mission, the pilot was found near the edge of a cliff. It was at this point that the large circler megalithic structure was discovered. 

A  stone circle different from the rest was discovered, it was aligned to the cardinal points of the rising and setting sun along with the equinoxes (an equinoxes is the instant of time when the plane of Earth's equator passes through the geometric centre of the Sun's disk) and solstices (a solstice is an event occurring when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere). Later studies showed the connection between it and the Orion constellation. 

The amount of erosion on the stone suggests a remarkable age of a possible 75,000 to 100,000 years old, giving it the name Adam's calendar as the oldest known megalithic structure on the planet.