Most ancient artifacts, no matter how strange they first appear, can eventually be explained and placed in their historical context by archaeologists and historians. The artifacts below are the exceptions to this rule. From mysterious metals to giant stone balls of amazing spherical precision, these objects remain perplexing to this day.

01 of 16 The Grooved Spheres Science informs us that man is a few million years old, and civilization just tens of thousands of years old. Could it be, however, that conventional science is mistaken? There is some evidence that the history of life on earth might be far different than what current geological and anthropological texts tell us. Consider these astonishing finds: Miners in South Africa have dug up mysterious metal spheres. Of unknown origin, these spheres measure approximately an inch in diameter, and some are etched with three parallel grooves running around the equator. Two types of spheres have been found: one is composed of a solid bluish metal with flecks of white, while the other is hollowed out and filled with a spongy white substance. The kicker is that the rock in which they where found is Precambrian—and dated to 2.8 billion years old. Who made them and for what purpose is unknown.

02 of 16 The Ica Stones Brattarb/Wikimedia Commons/CC SA BY 2.0 In the 1930s, Dr. Javier Cabrera received the gift of a strange stone from a local farmer. The stone so intrigued the doctor that he began collecting others, eventually boasting a collection of more than 1,100 andesite stones. Estimated to be between 500 and 1,500 years old, the stones have become known as the Ica Stones. The stones bear etchings, many of which are sexually graphic. Some depict idols, while others depict such practices as open-heart surgery and brain transplants. The most astonishing etchings, however, clearly represent dinosaurs—brontosauruses, triceratopses, stegosauruses, and pterosaurs. While skeptics consider the Ica Stones a hoax, their authenticity has neither been proved nor disproved.

03 of 16 The Antikythera Mechanism Tilemahos Efthimiadis//Wikimedia Commons/Public Domain A perplexing artifact was recovered by sponge-divers from a shipwreck in 1900 off the coast of Antikythera, a small island northwest of Crete. The divers brought up from the wreck a great many marble and bronze statues that had apparently been the ship's cargo. Among the findings was a hunk of corroded bronze that contained some kind of mechanism composed of many gears and wheels. Writing on the case indicated that it was made in 80 B.C., and many experts at first thought it was an astrolabe, an astronomer's tool. An x-ray of the mechanism, however, revealed it to be far more complex, containing a sophisticated system of differential gears. Gearing of this complexity was not known to exist until 1575. It is still unknown who constructed this amazing instrument 2,000 years ago or how the technology was lost.

04 of 16 The Baghdad Battery Boynton/Flickr.com/CC BY 2.0 Today, batteries can be found in any grocery, drug, convenience, and department store you come across. Well, here's a battery that's 2,000 years old. Known as the Baghdad Battery, this curiosity was found in the ruins of a Parthian village believed to date back to between 248 B.C. and 226 A.D. The device consists of a 5-1/2-inch clay vessel inside of which is a copper cylinder containing an oxidized iron rod. Experts have concluded that the device needed only to be filled with an acid or alkaline liquid to produce an electric charge. It is believed that this ancient battery might have been used for electroplating objects with gold. If so, how was this technology lost...and the battery not rediscovered for another 1,800 years?

05 of 16 The Coso Artifact While mineral hunting in the mountains of California near Olancha during the winter of 1961, Wallace Lane, Virginia Maxey, and Mike Mikesell found a rock that they thought was a geode—a good addition for their gem shop. Upon cutting it open, however, Mikesell found an object inside that seemed to be made of white porcelain. In the center was a shaft of shiny metal. Experts estimated that, if this was a geode, it should have taken about 500,000 years for the fossil-encrusted nodule to form, yet the object inside was obviously of sophisticated human manufacture. Further investigation revealed that the porcelain was surrounded by a hexagonal casing, and an x-ray revealed a tiny spring at one end, like a spark plug. There's a bit of controversy around this artifact, as you can imagine. Some contend that the artifact was not inside a geode at all, but encased in hardened clay. The artifact itself has been identified by experts as a 1920s-era Champion spark plug. Unfortunately, the Coso Artifact has gone missing and cannot be thoroughly examined. Is there a natural explanation for it? Or was it found, as the discoverer claimed, inside a geode? If so, how could a 1920s spark plug end up inside a 500,000-year-old rock?

06 of 16 Ancient Model Aircraft Olek95/Wikimedia Commons/Public Domain There are artifacts belonging to ancient Egyptian and Central American cultures that look amazingly like modern-day aircraft. This Egyptian artifact, found in a tomb at Saqquara in 1898, is a 6-inch wooden object that strongly resembles a model airplane, with fuselage, wings, and tail. Experts believe the object is so aerodynamic that it is actually able to glide. Estimated to be 1,000 years old, the small object is made of gold and could easily be mistaken for a model of a delta-wing aircraft—or even the Space Shuttle. It even features what looks like a pilot's seat.

07 of 16 Giant Stone Balls of Costa Rica Matthewobrien/Wikimedia Commons/CC BY SA 3.0 Workmen hacking and burning their way through the dense jungle of Costa Rica to clear an area for banana plantations in the 1930s stumbled upon some incredible objects: dozens of stone balls, many of which were perfectly spherical. They varied in size from as small as a tennis ball to an astonishing 8 feet in diameter. Although the great stone balls are clearly man-made, it is unknown who made them, for what purpose, and, most puzzling, how the creator(s) achieved such spherical precision.

08 of 16 Impossible Fossils Fossils, as we learned in grade school, appear in rocks that were formed many thousands of years ago. Yet there are a number of fossils that just don't make geological or historical sense. A fossil of a human handprint, for example, was found in limestone estimated to be 110 million years old. What appears to be a fossilized human finger found in the Canadian Arctic also dates back 100 to 110 million years ago. And what appears to be the fossil of a human footprint, possibly wearing a sandal, was found near Delta, Utah, in a shale deposit estimated to be 300 million to 600 million years old.

09 of 16 Out-of-Place Metal Objects Humans were not even around 65 million years ago, never mind humans who knew how to work metal. So how does science explain semi-ovoid metallic tubes dug out of 65-million-year-old Cretaceous chalk in France? In 1885, a block of coal was broken open to reveal a metal cube obviously worked by intelligent hands. In 1912, employees at an electric plant broke apart a large chunk of coal out of which fell an iron pot. A nail was found embedded in a sandstone block from the Mesozoic Era. And there are many, many more such anomalies.

10 of 16 Shoe Print in Granite This shoe print fossil was discovered in a seam of coal in Fisher Canyon, Nevada. The coal is believed to be 15 million years old. Close-up examination of the fossil reveals traces of a double line of stitches around the perimeter. It's about a size 13, and the right side of the heel appears to be more worn down than the left. How does a modern shoe print become impressed in 15-million-year-old material? Either the imprint was made recently and coal does not really take millions of years to form (an explanation unsupported by science), there were people (or something like people) walking around in shoes 15 million years ago (of which we have no historical record), time travelers went back in time and carelessly left this show print, or it is all an elaborate hoax. The jury is still out.

11 of 16 Ancient Footprint You could see a human footprint like this today on any beach or patch of mud. But this footprint—clearly from the anatomy of a modern human—is fossilized in stone estimated to be about 290 million years old. The discovery was made in New Mexico by paleontologist Jerry MacDonald in 1987. There were also fossil footprints of birds and other animals, but MacDonald was at a loss to explain how this modern footprint could possibly have been cast in Permian strata, which dates from 290 to 248 million years ago—long before man (or even birds and dinosaurs for that matter) existed on this planet. In an article that Smithsonian Magazine ran in 1992 about the discovery, it was noted that paleontologists call such anomalies as "problematica." Big problems indeed for scientists.

12 of 16 Ancient Springs, Screws, and Metal They look like objects you'd find in any workshop or machine shop scrap bin. They are obviously manufactured. Yet this assortment of metal springs, eyelets, spirals, and other metal objects was found in layers of sediment dated up to 100,000 years old. There weren't many metal foundries in those days. Thousands of these things—some as small as 1/10,000th of an inch—were discovered by gold miners in the Ural Mountains of Russia in the 1990s. Dug up from a depth of three to 40 feet in layers of earth dating back to the upper Pleistocene era, these curious objects could be anywhere from 20,000 to 100,000 years old. Are they evidence of a long-lost but advanced civilization?

13 of 16 Metal Rod Encased in Stone How can we explain a stone that appears to have formed around a mysterious metal rod? Found by rock collector Zhilin Wang in the Mazong Mountains of China, the hard black rock has embedded within it a metal rod of unknown origin and purpose. The rod has screw-like threads, suggesting that it was manufactured, yet the fact that it was in the ground long enough for hard rock to form around it means that it must be millions of years old. It has even been suggested that the rock is a meteorite that fell to Earth from space, making it extraterrestrial in origin.

14 of 16 Aiud Aluminum Artifact This 5-pound, 8-inch object of solid, nearly pure aluminum was found in Romania in 1974. Workers digging a trench along the Mures River discovered a couple of mastodon bones and this enigmatic object, over which scientists have puzzled ever since. Clearly manufactured, the artifact was sent for analysis and was found to be composed of 89 percent aluminum with traces of copper, zinc, lead, cadmium, nickel, and other elements. Aluminum in this form is not found free in nature, but must be manufactured and wasn't produced in quantity until the 1800s. If it is the same age as the mastodon bones, that would make it at least 11,000 years old, when the last of that species went extinct. An analysis of the oxidized layer coating the artifact dated it to 300 to 400 years old—still well before the aluminum manufacturing process was invented. Who made this object? And what was it used for? There are those who are quick, of course, to suggest it is of extraterrestrial origin...but the facts are still unknown.

15 of 16 The Piri Reis Map Heritage Images/Getty Images / Getty Images This map, rediscovered in 1929 in a Turkish museum, is a puzzlement, not only for its remarkable accuracy, but also for what it shows. Drawn on gazelle skin, the Piri Reis map is part of a larger map, but only one half has survived. It was compiled in the 1500s from, according to writing on the map itself, other maps dating back to around the year 300. But how can this be, given that the map shows a partial view of a southern continent where we now know Antarctica to be (Antarctica was not discovered until 1820)?