1844年，一群测量员在密歇根上半岛发现，指南针竟然出现异常，而背后的原因是一种记录着远古地球秘密的岩石。科学家如何通过岩石、陨石和放射性元素，测出地球已有46亿年的历史？
In 1844, a group of government surveyors discovered something… weird.
They were trudging through Michigan's Upper Peninsula, toward the shore of Lake Superior, when suddenly, their compasses went haywire.
Now, I've seen enough horror movies for this to raise some red flags.
When the compasses get funky, there's something funkier afoot.
What the surveyors found was a big surprise, though not a supernatural one.
It was a whole lot … of iron.
And that iron would change what we know about the history of the world.
Hi!
I'm Sage, and this is Crash Course Geology.
To get started, we have to turn back the clock.
