I have been hearing a lot about this over the past few years.....the Bosnian Pyramid Of The Sun is estimated to be one and a half times the size of the main pyramid at Giza...and possibly 25,000 years old...
And it is only one of several very ancient pyramids in the area...
http://www.bosnianpyramids.org/index.php?id=36&lang=en
http://www.bosnianpyramids.org/index.php?id=31&lang=en