So how many of you have ever been in a cave（山洞） before? Okay（好）, a few of you. When you think of a cave, most of you think of a tunnel（隧道） going through solid（固体的） rock. In fact, that's how most caves are. Around this half of the country, most of your caves are made of limestone（[岩] 石灰岩）. Back where I'm from, most of our caves are made of lava（熔岩） rock, 'cause we have a lot of volcanoes out there.

  But the caves I want to share with you today are made completely of ice, specifically（明确地） glacier ice, that's formed in the side of the tallest mountain in the state of Oregon（俄勒冈州(美国州名)）, called Mount（山峰） Hood（头巾）. Now, Mount Hood's only one hour's drive from Portland, the largest city in Oregon, where over two million people live. Now, the most exciting thing for a cave explorer is to find（找到） a new cave（山洞） and be the first human to ever go into it. The second most exciting thing for a cave explorer is to be the first one to make a map of a cave. Now, these days, with so many people hiking（远足） around, it's pretty hard to find a new cave. So you can imagine how excited we were to find three new caves within（在……之内） the site of Oregon（俄勒冈州(美国州名)）'s largest city and realize that they had never been explored（探险） or mapped before.

  It was kind（种类） of like being an astronaut, 'cause we were getting to see things and go places that no one had ever seen or gone before. So what is a glacier（冰河）? Well, those of you that have ever seen or touched snow, you know that it's really light, 'cause it's just a bunch（束） of tiny（微小的） ice crystals（结晶） clumped together, and it's mostly air. If you squish a handful（一把） of snow to make a snowball, it gets really small, hard, and dense（密集的）. Well, on a mountain-like hood where it snows over 20 feet a year, it crushes（压碎） the air out of it and gradually（逐渐地） forms it into hard blue ice. Now, each year, more and more ice stacks（堆） up on top of it and eventually（终于） gets so heavy that it starts to slide（滑） down the mountain under its own weight, forming a slow-moving river of ice.

  When an ice pack（包） like that starts to move, we call it a glacier and we give it a name. The name of the glacier these caves are forming is the Sandy（(Sandy)人名） Glacier（冰河）. Now, each year, as new snow lands on the glacier, it melts（（使）融化） into summer sun, and it forms little rivers of water that flow（流动） along the ice, and they start to melt and bore（使厌烦） their way down through the glacier（冰河）, forming big networks of caves, sometimes going all the way down to the underlying（成为……的基础） bedrock. Now, the crazy thing about glacier caves is that each year, new tunnels form, different waterfalls（瀑布） pop（ 发出“砰”的一声） up or move around from place to place inside the cave（山洞）. Warm water from the top of the ice is boring its way down, and warm air from below the mountain actually rises up, gets into the cave and melts the ceilings（天花板） back taller and taller. But the weirdest（怪异的） thing about glacier（冰河） caves（山洞） is that the entire cave（山洞） is moving, because it's formed inside a block（街区） of ice the size of a small city that's slowly sliding（滑） down the mountain.

  Now, this is Brent McGregor, my cave exploration（探险） partner.