The Nebra Sky Disk

In 2001, this mysterious artifact showed up on the international antiquities market, and the man attempting to auction it off foolishly and naively made it no secret that it had been dug up from a Saxony-Anhalt gravesite in Nebra by amateur treasure hunters in 1999.

What the man apparently didn't realize was that what he did was illegal. But even if he had obtained the disk by legal means, all archaeological artifacts are the property of the state in Saxony-Anhalt. The disk was seized by police and placed in the hands of the state archaeologist, Dr Harald Meller.

Wikipedia:

As part of a plea bargain, the illicit owners led police and archaeologists to the site where they had found it together with other remains (two bronze swords, two hatchets, a chisel and fragments of spiral bracelets). Though no witnesses were present at the first discovery, archaeologists have opened a dig at the site and have uncovered evidence that support the looters' claim (in the form of traces of bronze artifacts in the ground, as well as matching earth samples found sticking to the artifacts)...

The discovery site identified by the arrested metal detectorists is a prehistoric enclosure encircling the top of a 252 m elevation in the Ziegelroda Forest, known as Mittelberg ("central hill"), some 60 km west of Leipzig. The surrounding area is known to have been settled since the Neolithic, and Ziegelroda Forest is said to contain around 1,000 barrows.

The enclosure is oriented in such a way that the sun seems to set every solstice behind the Brocken, the highest peak of the Harz mountains, some 80 km to the northwest. It was claimed by the treasure-hunters that the artifacts were discovered within a pit inside the bank-and-ditch enclosure. The signifiance of the site to prehistoric dwellers is underlined by the proximity to the much older Goseck circle.

Like Stonehenge and the Antikythera Mechanism, the Nebra Sky Disk appears to be an astronomical instrument that demonstrates greater knowledge of the solar calendar and stellar movements on the part of its creators than previously supposed.

A documentary about the artifact, Secrets of the Star Disc, airs Monday, March 1, 1pm on the National Geographic Channel.