Mysterious stone discs discovered in Ukraine may be examples of advanced Viking technology / Archaeologists unearth 150-year secret in major Viking breakthrough / Rare Viking-age treasure begins international tour / What archaeologists are learning from the discovery of 50 extremely rare Viking skulls / Archaeologists discover huge treasure trove including 50 extremely rare Viking skulls / Well-preserved Viking skeletons discovered in Denmark / Archaeologists discover more than 50 exceptionally well-preserved Viking skeletons in Denmark