This ancient ‘portal’ was created out of a single Andesite block weighing approximately 10 tons and is around 9.8 ft. (3.0 m) tall and 13 ft. (4.0 m) wide. Some authors argue that it dates back to around 15,000 BC.
Located in the vicinity of one of the most marvelous lakes on Earth—Lake Titicaca—The gate of the sun was constructed thousands of years ago by an ancient culture inhabiting modern-day Bolivia, at the archaeological site of Tiahuanaco.
Ever since European explorers stumbled across the enigmatic strcuture—archaeologists, historians, and tourists have been left puzzled by the Gate of the Sun.
Among experts, it is known as one of the best examples of the degree of perfection that its building culture reached, both for the art and for the symbolism that treasures its reliefs.
Most experts agree that this ‘portal’ was created out of a single Andesite block weighing approximately 10 tons, and is approximately 9.8 ft. (3.0 m) tall and 13 ft. (4.0 m) wide. 
In the past, the Gate of the Sun was part of a larger building that was most likely located at the top of the Pyramid of Akapana or at the Kalasasaya temple, where there are several constructions that are made out of the same material as the Gate of the Sun.
The Gate of the sun has curious depictions on its surface, the most recognizable is that of Viracocha, who is depicted surrounded by winged creatures who kneel before him. Are those the depictions of the same winged creatures we see in ancient Mesopotamia? There are 32 effigies with human faces and 16 with condors’ heads.
Viracocha is the great creator deity in the pre-Inca and Inca mythology in the Andes region of South America. According to legend, Viracocha rose from Lake Titicaca during the time of darkness to bring forth light and created the sun moon and the stars. Curiously, this ancient deity is said to have CREATED mankind by breathing into stones, but his first creation were brainless giants that displeased him. So he destroyed it with a flood and made a new, better one from smaller stones.
The door was never finished as experts have found several unfinished figures as if the carvers had hurriedly left behind one of their most magnificent structures.
The Gate of the sun has in its upper-right a crack that divides the stone into two. It is believed to have been struck by lightning, but this theory can not be verified because there are no burn marks on the stone or inside it.
If we take a look at some of the legends that mention giants, the door was cracked as it was thrown to the ground, cracking the gate of the sun into two.
This remains a mystery to archaeologists.
According to Aymara legend, the door holds a secret that the ancient Aymaras left hidden to help future humanity in trouble.
It is currently one of the most important monuments of Tiahuanaco.
Tiahauanco—the city complex—was the center of the ancient Tiahuanaco civilization, an extremely well-developed pre-Inca culture that based its economy on agriculture, architecture, and livestock. The ancient culture covered the territories of the Collao plateau, located between southwest Peru, western Bolivia, north of Argentina and northern parts of Chile, regions from which the culture passed on its technological and religious influence towards other contemporary civilizations.
The city of Tiahuanaco is characterized by its incredible intricate architectural style, decorated with countless reliefs, which in turn are shrouded in mystery.
The city complex is composed of several important architectural constructions: the Akapana, Akapana East, and Pumapunku stepped platforms, the Kalasasaya, the Kheri Kala, Putuni enclosures, and the Semi-Subterranean Temple, Puerta del Sol (Gateway of the Sun) and Puma Punku which in turn has countless structures that defy mainstream archaeologists.
As noted by recent studies, these architectural examples, as well as the Akapana Gate, have unique details and demonstrate high skill in stone-cutting. This reveals a knowledge of descriptive geometry. The regularity of elements suggests they are part of a system of proportions.
It is believed that Tiahuanaco is one of the oldest cultures of South America, having survived from around 1500 B.C. until the year 1000 A.D
The general consensus is that the ancient Tiahauanco culture began around 1580 B.C.
This ancient culture is characterized by having bronze tools and weapons, which gave it a great technological and military advantage with respect to the other cultures in America during that time in history; Proof of this is that brass unions have been found in their constructions, and several tools and weapons have been unearthed by experts.
Some authors—like Arthur Posnansky—estimated that Tiwanaku dated to 15,000 BC, based on his archaeoastronomical techniques. 
Posnansky’s theory states that after the great flood of around 11,000 BC, the survivors developed advanced techniques of agriculture that only today are being studied by certain experts. In particular, this ancient culture succeeded in obtaining, by means of crosses and techniques that until now are not well understood, excess production of maize, quinoa, kiwicha, potatoes and maca (really powerful nutrient still used today), although the climate had been cooled down excessively. In his famous book “Tiahuanaco, the cradle of American man“, Posnansky proposes two theories that according to him are essential for considering Tiahuanaco as the oldest city on the surface of the planet.
Given its apparent antiquity, some scholars proposed that Tiahuanaco was the mother culture of all South American civilizations, while others regarded it as the capital of an ancient megalithic empire or a great empire that spread throughout the Central Andes.
More from Brien Foerster:
 Fernando Cajías de la Vega, La enseñanza de la historia : Bolivia, Convenio Andrés Bello, 1999,p.44.