Most are sculpted from granodiorite, an igneous rock similar to granite. They are believed to have been created between 200 BC and 1600 BC. The stones are often grouped together in geometrical patterns, often pointing to magnetic north. Local people call those spheres 'Las Bolas' and they can be found in many parts of Costa Rica.
The common belief is that the Spheres are perfectly spherical, however, laser analysis has shown that they are only 96% perfect. This is still pretty impressive levels of accuracy.
The spheres were discovered in 1930 while the United Fruit Company was clearing land for a new plantation. The orbs were identified as man-made and moved out the way with heavy machinery. Soon after, the workers learned of local legends saying that the spheres were filled with gold, so they returned and dynamited a lot of them. Those that survived were transported around the world as curios.
Mystery surrounds the creation of these spheres due to their size and almost perfect shape. The rock they are carved from is particularly hard, so exactly what tools were used to create them is also a mystery. One myth is that the native inhabitants had access to a chemical able to soften the rock.
Another mystery is how the larger spheres were transported. It is widely accepted that large stone can be moved by teams of people using rope, rollers, grease and a lot of manpower, however, the spherical shape of these orbs would mean that this form of transportation is difficult.