What do the Eiffel Tower, the Taj Mahal, and the Golden Gate Bridge have in common? Besides being one of the eight wonders of the world, they're also man-made structures. Humans have consistently proven their intelligence and evolution in technology through their construction of epic projects.

However, humanity has been known for its fair share of mystery. Many pre-historic civilizations have created structures that seem to defy reason and, if the Egyptian pyramids are any indication, even human strength!

Archeologists have made it their life's work to unearth the mysteries of these structures and retrieve as many clues as they can about those that came before us. Here are twenty structures, discovered by ordinary people, that have still yet to be explained.

20 Puma Punku

This Bolivian rock structure has puzzled archeologists for years. Said to date back to 536, experts don't understand how it could have been created without the use of specialized tools or technology. The largest stone weighs about 130 metric tons, leading experts to believe that these stones are not man-made.

19 Newgrange

Newgrange is Ireland's version of Stonehenge. Built over 5000 years ago, it is older than both Stonehenge and even the Pyramids! Located in Brú na Bóinne and made up of four underground tombs, experts believe that this structure was built to represent the four most important days of the year, including the solstice.

18 Giant Stone Spheres (Yup, That Simple!)

Though these structures are simple in nature, they are no less mysterious. Known by Costa Rican locals as "Las Bolas," they are said to be the remnants of pre-Columbian society. Some suggest that they were used for astronomical purposes, while others suggest that they were used as signposts.

17 Quin Shi Huang Tomb

Who says you can't be protected after you pass? This army of terracotta warriors was discovered by Chinese farmers in 1974. Emperor Qin Shi Huang had these built to protect him in the afterlife. This fact is well known by historians. The real mystery is where the Emperor is actually buried!

16 The Great Pyramids

The pyramids have been a staple of Egypt for centuries. The 5000-year old structures represent the ancient Egyptian's reverence towards their Pharaohs, as well as their culture. The mystery lies in how and why these pyramids were built in the first place. Archeologists are still making new discoveries about them today.

15 Gobekli Tepe

These ruins, discovered in rural Turkey, have led many to rethink the evolution of civilization! Carvings of animals were discovered on the pillars within the structure, indicating that this is one of the oldest places of worship. However, the builders were said to be semi-nomadic. So what came first? Building the structure or settling down?

14 Super-Henge

No, this isn't a joke! This formation of massive stones was discovered just 2 miles from the original Stonehenge. According to experts, this monument was used by Neolithic people. Its purpose, however, is still largely unknown. They assume, however, that the stones used to stand upright before being tipped over.

13 Cochno Stone

This massive stone slab was discovered in Glasgow, Scotland, in 2016. Although mystery surrounds it, archeologists have a few hypotheses. The "cup and rink marks" on it are indicative that this 5000-year-old slab might be prehistoric artwork. The inscriptions are also thought to be linked to astronomical occurrences.

12 Cahokia Mounds

This group of earthen mounds is a remnant of old Indigenous camps. Experts believe that nomadic tribes occupied this site for 700 years. At its peak, 100,000 people occupied the space. The mystery of the area, however, lies in why this tiny city became abandoned. Even local folklore is puzzled as to why.

11 Big-Horn Medicine Wheel

Located on the Big Horn Range in Wyoming, the accuracy of this giant medicine wheel is what makes it so puzzling. It was thought to be built to identify the position of the stars and the sun during the summer solstice. Many Indigenous tribes use it today for ceremonies and rituals.

10 The Great Serpent Mound

This mound, measuring 1300 feet in length, has been quite the head-scratcher. It has been attributed to the Fort Ancient Culture, an Indigenous group of the first century. The serpent has supernatural connotations within Indigenous folklore. It is also said that it was used to mark the seasons.

9 Stonehenge

Stonehenge has rightly earned its place as one of the world's great mysteries. The only thing that archeologists know for sure about this stone formation is that it was built 4000 years ago. Besides, no one is quite sure what its purpose was. Theories range from an observatory to a temple.

8 Berkeley "Mystery" Walls

Traveling fifty miles down the coast of the East Bay, this mysterious rock wall has been standing before settlers arrived. The most mysterious thing about this wall is that it seems to serve no purpose and was not correctly built to act as an enclosure or a defense mechanism.

7 Miami Circle

While a luxury condominium complex was being built in downtown Miami, a large circle was discovered. Stranger still, there were twenty-four perfect circles within the latter's diameter. Many artifacts have been recovered, including tools belonging to the Tequesta tribe. Besides that, no one knows the exact purpose of the formation.

6 Coral Castle

This little art exhibit was built by Ed Leedskalnin as a tribute to an old lover. People are quite hard-pressed to understand how he achieved this without the use of machinery. Here's the spooky part. When asked, he simply stated that he knew the ways of building the pyramids.

5 Chaco Canyon

This canyon has many indications that ancient civilizations inhabited the area. They built massive houses out of stone, some four stories high and containing almost 800 rooms! The mystery lies in its abandonment. Some areas show signs of being attacked, but other areas were purposefully sealed. It indicates that the people meant to return.

4 The Grave of The Female Stranger

As legend has it, a man attempting to help his ill wife wouldn't allow the doctors to reveal her identity. After she passed, he had her secretly buried. Her tombstone refers to her as "a Female Stranger." Some speculate that this was Aaron Burr's daughter, Theodosia, who was lost at sea.

3 Sanxingdui Stone Artifact

This treasure wasn't discovered by archeologists, but a Chinese sewage worker! Made out of jade and stone, this artifact is thought to have been created by the Sanxingdui civilization over 3000 years ago. They were said to inhabit a walled city along the Minjian River and they routinely buried their treasures along its banks.

2 Eternal Flame Falls

Though this isn't a rare phenomenon, the location of this particular flame is what makes it strange. Situated behind a waterfall in Orchard Park, New York, the water should make it difficult for the flame to keep burning. According to scientists, the flame contains more ethane and propane than your average flame.

1 Georgia Guidestones

A mysterious man named RC Christian commissioned the building of these stones back in 1979. Inscribed on the slabs is a message to humanity. It implores them to, among many things, "Maintain humanity under 500,000,000 in perpetual balance with nature." Christian disappeared after commissioning the project.