I remember being profoundly amazed that anyone would suppose that a little wooden desk would provide a safe haven in the event of an atomic bomb bein… - Bill Bryson

The upshot of all this is that we live in a universe whose age we can't quite compute, surrounded by stars whose distances we don't altogether know, filled with matter we can't identify, operating in conformance with physical laws whose properties we don’t truly understand.

How do migrating birds know which one to follow? What if the lead bird just wants to be alone?

Distance changes utterly when you take the world on foot. A mile becomes a long way, two miles literally considerable, ten miles whopping, fifty miles at the very limits of conception. The world, you realize, is enormous in a way that only you and a small community of fellow hikers know. Planetary scale is your little secret.

Life takes on a neat simplicity, too. Time ceases to have any meaning. When it is dark, you go to bed, and when it is light again you get up, and everything in between is just in between. It’s quite wonderful, really.

You have no engagements, commitments, obligations, or duties; no special ambitions and only the smallest, least complicated of wants; you exist in a tranquil tedium, serenely beyond the reach of exasperation, “far removed from the seats of strife,” as the early explorer and botanist William Bartram put it. All that is required of you is a willingness to trudge.

There is no point in hurrying because you are not actually going anywhere. However far or long you plod, you are always in the same place: in the woods. It’s where you were yesterday, where you will be tomorrow. The woods is one boundless singularity. Every bend in the path presents a prospect indistinguishable from every other, every glimpse into the trees the same tangled mass. For all you know, your route could describe a very large, pointless circle. In a way, it would hardly matter.

At times, you become almost certain that you slabbed this hillside three days ago, crossed this stream yesterday, clambered over this fallen tree at least twice today already. But most of the time you don’t think. No point. Instead, you exist in a kind of mobile Zen mode, your brain like a balloon tethered with string, accompanying but not actually part of the body below. Walking for hours and miles becomes as automatic, as unremarkable, as breathing. At the end of the day you don’t think, “Hey, I did sixteen miles today,” any more than you think, “Hey, I took eight-thousand breaths tod