Voorhies sent samples to colleagues all over the western United States asking if there was anything about it that they recognized. Several months later a geologist named Bill Bonnichsen from the Idaho Geological Survey got in touch and told him that the ash matched a volcanic deposit from a place called Bruneau-Jarbidge in southwest Idaho. The event that killed the plains animals of Nebraska was a volcanic explosion on a scale previously unimagined—but big enough to leave an ash layer ten feet deep almost a thousand miles away in eastern Nebraska. It turned out that under the western United States there was a huge cauldron of magma, a colossal volcanic hot spot, which erupted cataclysmically every 600,000 years or so. The last such eruption was just over 600,000 years ago. The hot spot is still there. These days we call it Yellowstone National Park.
沃里斯把样品寄给美国西部各地的同事们，问他们是否认得出这是什么东西。几个月以后，爱达荷州地质勘测局的一位名叫比尔·邦尼奇森的地质学家跟他取得联系，告诉他这种灰末与一种火山沉积物完全吻合，来自爱达荷州西南部一个名叫布鲁诺-贾比奇的地方。那个使内布拉斯加平原上的动物死于非命的事件是一次火山爆发，其规模以前没有想像过──但足以在1600公里以外的内布拉斯加东部地区留下3米厚的一层火山灰。结果证明，美国西部的下面有一大片岩浆，一个巨大的火山热点。它每隔60万年左右灾难性地喷发一次。最近一次这样的喷发就在60多万年以前。那个热点仍在那里。如今，我们称其为黄石国家公园。

We know amazingly little about what happens beneath our feet. It is fairly remarkable to think that Ford has been building cars and baseball has been playing World Series for longer than we have known that the Earth has a core. And of course the idea that the continents move about on the surface like lily pads has been common wisdom for much less than a generation. "Strange as it may seem," wrote Richard Feynman, "we understand the distribution of matter in the interior of the Sun far better than we understand the interior of the Earth."
我们对自己脚底下在发生的事知道得实在太少。福特公司开始生产汽车，棒球世界联赛开始举办，其时间比我们知道地球有个地核还要长，想到这点真令人觉得怪有意思。当然，人们知道大陆在地球表面像浮在水面上的睡莲叶子似的到处移动的事，还远不到一代人的时间。“尽管不可思议，”理查德·费曼写道，“我们对太阳内部的物质分布的认识，远比对地球内部的认识要多。”