But either of those scenarios -- in fact, just about anything that would seem extreme to me -- would probably sound like a luxury vacation to most of the envelope-pushing adventurers Frances Ashcroft writes about in her new book, "Life at the Extremes: The Science of Survival."

Ashcroft, a professor of physiology at Oxford University in England, combines personal narrative, tales of derring-do and, most of all, accessible scientific explanations to describe what happens when human beings push themselves to their limits -- when they succumb to the overpowering urge to climb high, dive deep, run fast, immerse themselves in unbearable heat or cold and venture into the mysterious and dangerous realm of space.

She writes of men and women who have succeeded, and of those who have failed. She answers the questions that have plagued you for years, like: Why don't sperm whales get the bends when human divers do? Why can some human beings climb Mount Everest without extra oxygen while passengers in a plane that depressurizes at the same altitude stand no chance of survival? And what is the Vomit Comet?

And she writes of scientists and researchers who sometimes put their lives on the line (usually because they couldn't convince anybody else to do so) to solve some of the riddles of human survival. There was, for example, Sir Charles Bladgeon, who, in the 18th century, walked into a room heated to 105 degrees C (221 F) with some eggs, raw steak and a dog. Within 15 minutes, the steak and eggs were cooked to perfection; Bladgeon and the dog, fortunately, were not.

Ashcroft herself is no stick-in-the-mud. She has experienced some of the extremes she documents, including immersion in superhot Japanese baths, altitude sickness climbing Mount Kilimanjaro and the terror of her first deep-sea dive. Talking with Salon by telephone from London must have seemed like child's play by comparison.

The book's cover has a sort of eerie photo of diving women. What's that about?

Ah, yes, the Ama women of Japan. They've been around for 2,000 years. It's such an onerous profession. They're diving for abalone and shellfish and for the mother-of-pearl shells. The highly trained ones, the funado, dive to about 20 meters [66 feet] deep. They have a large weight attached that enables them to go down to the bottom. They dive down with a rope tied around their waist, and they have a small basket that they fill with the prime delicacies of Japan. And then a male assistant hauls up the basket, and the woman may fill more baskets, depending how long she can hold her breath. Then she's pulled up to the top by the assistant. At that depth, you do need someone to help you up, because as you dive, the air in your lungs compresses and you tend to sink.

One lady-in-waiting at the court of the Japanese empress a thousand years ago described the fact that the women are the ones doing all the work. They are diving down into the cold, having to hold their breath, and then come up sputtering and choking. Meanwhile, the men who are their partners sit in their little boats on the surface of the water, drinking and telling jokes and laughing and talking to all the other men, and generally having a good time.

Why do some of us prefer the comfortable middle, and others seem to have a real thirst for life at the extremes?

I think that there are two different sorts of people -- those who do it for the kicks, the adrenaline thrill, and those who do it because they find it interesting and fascinating. I'm one of the latter. The challenge of going up a mountain or looking at landscapes you've never seen before, is just exhilarating. I'd love to go into space and look down on Earth and space, wouldn't you?

No, I think it would be quite frightening! Talk about some of the extremes that you have gone to.

Personally speaking, I'm a wimp. I do things I think are interesting and exciting, but I have never really put myself in any danger. I have climbed to the top of Kilimanjaro. I have done a little scuba diving. I have been in very cold and very hot environments. But we all put ourselves in conditions where there are physical challenges to us; it's just that we don't necessarily notice them. All of us fly in airplanes, and if the plane depressurized, we would have a big problem. There are many things that we need to do, and we have engineered our environment so that we can survive without difficulty.

Without meaning any disrespect, what's exhilarating about feeling, on the top of Kilimanjaro, like your head's going to blow off from altitude sickness, as you describe in the book? To be honest, when you are in an extreme situation, you're not really thinking of the scientific explanations, are you? Aren't you just thinking, Get me out of here?

If you're a scientist, you see things on many different levels. I do think, I don't want to be here; let's get out of here. But at the same time, my brain is saying, What's going on, what's working, what can I do to make this better, how can I behave? And at the top of Kilimanjaro, the view is wonderful! There you are, and you have ice fields on the equator!

Is there an extreme you wouldn't go to?

The people I admire most of all are firefighters. I'm afraid of fire. The whole idea of walking into a firestorm -- that to me is so scary I just couldn't bear to do it. My recurrent nightmare is running down a road engulfed in flames.