China's first emperor, Qin Shi Huang (died 210 BC), lies under a tree-covered mound near Xi'an that has never been opened. The Terracotta Army was buried to guard it.

• ✓Sima Qian, Shiji (Watson trans.)
• ✓Zhao et al. 2020, Scientific Reports 10:10414
• ✓Ball 2015, Chemistry World
• ✓Wikipedia, "Mausoleum of the First Qin Emperor"
• ✓Wikipedia, "Ding Mausoleum"

No one has been inside this hill for more than two thousand years. It's the tomb of China's first emperor, near the city of Xi'an, the man the Terracotta Army was built to guard. About a century after he died, China's most famous historian described what's inside: a palace, crossbows set to fire at intruders, and rivers and seas made of mercury. For centuries, no one could check whether any of it was true. Then, in the nineteen-eighties, scientists tested the soil on the hill and found up to fifty times more mercury than in the land around it. In twenty sixteen, lasers found mercury in the air above the mound too, and the scientists think it's escaping from the buried tomb through cracks. China won't dig it up. When they uncovered the Terracotta Army, its painted colours flaked off within minutes. So the best clue to what's in this hill is what's leaking out of it. Subscribe for a fact every day.