In the 1960s，while studying the volcanic history of Yellowstone National Park，Bob Christiansen became puzzled about something that，oddly，had not troubled anyone before：he couldn't find the park's volcano. It had been known for a long time that Yellowstone was volcanic in nature — that's what accounted for all its hot springs and other steamy features. But Christiansen couldn't find the Yellowstone volcano anywhere.
20世紀60年代，鮑勃·克裡斯琴森研究黃石國家公園火山歷史的時候，有個問題讓他緊鎖眉頭：他怎麼也找不到公園的火山口。說來也怪，以前向來沒有因為這樣的事麻煩過別人。從本質上講，黃石就是火山噴發巖，這是人所共知，由來已久的事——這也恰恰可以解釋為什麼這裡享有豐富的溫泉資源以及其它水氣氤氳的地貌特徵。可是，克裡斯琴森就偏偏找不到黃石火山口的位置。
Most of us，when we talk about volcanoes，think of the classic cone shapes of a Fuji or Kilimanjaro，which are created when erupting magma piles up. These can form remarkably quickly. In 1943，a Mexican farmer was surprised to see smoke rising from a small part of his land. In one week he was the confused owner of a cone five hundred feet high. Within two years it had topped out at almost fourteen hundred feet and was more than half a mile across. Altogether there are some ten thousand of these volcanoes on Earth，all but a few hundred of them extinct. There is，however，a second less known type of volcano that doesn't involve mountain building. These are volcanoes so explosive that they burst open in a single big crack，leaving behind a vast hole，the caldera. Yellowstone obviously was of this second type，but Christiansen couldn't find the caldera anywhere.
我們一談起火山，大多數想到的都是那種經典的錐形火山，比如富士山和吉力馬札羅山，它們可能是由噴出的巖漿轉瞬堆積而成。1943年，有一位墨西哥農民看到他家地裡有一小塊區域青煙嫋嫋就很納悶。不到一周時間，地上就隆起來一座500英尺高的小山丘。不足兩年山丘拔地而起高達差不多1,400英尺，覆蓋地表直徑半英裡還多。世界上這樣的火山大約有上萬座，除幾百座之外都已成為死火山。不過還有一種鮮為人知的火山，並沒有成山。這些火山在噴發時相當劇烈，會撕開一道巨大的裂縫，最後遺留下巨型的火山坑，也叫火山口。顯然，黃石火山屬於後者，但是克裡斯琴森怎麼也找不到火山口究竟在哪裡。
Just at this time NASA decided to test some new high-altitude cameras by taking photographs of Yellowstone. A thoughtful official passed on some of the copies to the park authorities on the assumption that they might make a nice blow-up for one of the visitors' centers. As soon as Christiansen saw the photos，he realized why he had failed to spot the caldera; almost the whole park-2.2 million acres—was caldera. The explosion had left a hole more than forty miles across—much too huge to be seen from anywhere at ground level. At some time in the past Yellowstone must have blown up with a violence far beyond the scale of anything known to humans.
恰在此時，美航空航天局決定要檢測幾部新型的高海拔攝像機，他們選擇對黃石火山進行高空拍攝。一位好心的官員考慮到公園管理部門的需要，順便留了幾張照片，放大後可供遊客接待中心使用。克裡斯琴森一看到照片就明白他為什麼找不到火山口了；幾乎整個220萬英畝的公園就是火山口。火山噴發遺留下的火山坑直徑超過40英裡——簡直太大了，從地平面任何地方根本看不出來。過去不知何時，這個地方肯定遭遇過火山爆發，其劇烈程度對人類來說還是個未知數。
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