The Wild File
Your urgent inquiries about the world, answered
By Hampton Sides

Is it true that Earth's topsoil is perpetually eroding? If so, why are older civilizations buried instead of exposed?
ù Rafael Rabines, San Diego, California

Consider it the great yin-and-yang of Earth science. While soil and rocks are steadily sloughing off and kicking up fine particles of dirt into the atmosphere, that same debris is just as faithfully returning to ground somewhere else. "The Buddhists have had it right all along," says Yale archaeologist Michael D. Coe. "Everything is in continuous motion. Mountains are being worn down, but new mountains are being formed. And no matter how hard you scrub away the windblown dirt, it just keeps raining down on you." In short, it's a zero-sum game of global proportions: One region's loss of sediment is another's gain. Not surprisingly, the biggest factors in tipping the sedimentary scale are climate and terrain. Mountainous areas are prone to steady winds and steep drainages and thus are forever losing ground ù literally. For archaeologists, this means that entire civilizations can pop up in sudden, unexpected ways, without anyone having to lift a single trowel of dirt. Take, for example, Central Asia's ancient Silk Route, where stout Mongolian winds have caused 1,500-year-old ruins such as the mysterious Kingdom of Loulan to rise like phantoms out of the desert. The breeze-strewn sand descends most often on protected, low-lying river valleys, leaving ancient treasures buried in perpetuity ù or at least until a team of archaeologists diligently digs them up. (Note the dozens of Roman monuments that have been uncovered in the Tiber River Valley.) Major forces of erosion aside, there's another reason that the older-is-deeper rule doesn't always apply. Earthquakes, floods, rivers that change course, and even burrowing rodents can move the archaeological furniture around to such an extent that the depth at which a given relic is discovered may not offer the faintest clue as to its age ù more compelling proof that there's no such thing as terra firma.

Whenever I hike for a long time, my hands get numb. What's wrong?
ù James White, Omaha, Nebraska

Fear not ù this is a common vexation that's both simply caused and simply alleviated. When you walk with your arms dangling at your sides for long periods, the steady pendulumlike motion creates just enough centrifugal force to drive the blood downward toward your hands ù and keep it there. Because your muscles aren't contracting enough to offset this pressure, the blood in your fingers has to fight gravity to make its way back to your heart. Thwarted, it pools in your hands, causing your fingers to become swollen and tingly from lack of oxygen. The remedy? Make a fist, bend your elbows, raise your arms above your head, or just pick up the pace: Runners seldom suffer from numb fingers, thanks to much energetic arm-pumping.

Why are there so many worms on the streets in the morning after a big rainstorm?
ù Ken Younger, Bozeman, Montana

This is a woeful occurrence that constantly thins out the ranks of the annelid world. Because worms are made up of more than 80 percent water and have highly permeable skin, they can venture from their burrows only when it's dark and extremely humid outside. An evening shower, therefore, is an occasion of great excitement for red wigglers, night crawlers, and the rest of their ilk. Stimulated by changes in barometric pressure and enticed by the moist conditions on the surface, they emerge to frolic in the topsoil and gorge themselves on newly softened organic matter. It's a regular throwdown ù that is, until dawn. Worms, you see, can abide no more than half an hour of direct sunlight: The UV rays paralyze them, and they start to dry out. Furthermore, worms have no eyes (though they do leave behind a trail of signature slime), so those unlucky revelers that stray too far onto the pavement and lose their bearings find themselves wriggling blindly on a battlefield of concrete. Cars squish them. Skateboards roll over them. And the early bird has no shortage of victims.

Am I imagining this, or is garbage sometimes warm to the touch?
ù Reginald Dixon, Charleston, West Virginia

What you're feeling is the metabolic heat of millions of microbes feasting on the organic matter in your trash. Spores of bacteria and microscopic fungi are perpetually blowing through the air in a dormant state, but once they zero in on a dank pile of refuse ù be it an outdoor compost heap or a kitchen trash can ù they immediately go to work colonizing the stuff. Rubbish, you might say, is their medium, and nothing's too rank for their tastes: As long as the garbage is well ventilated, these aerobic microorganisms will reproduce wildly and their collective body heat will rise. Compost piles usually top off in the vicinity of 180 degrees Fahrenheit, at which point the bacteria start to roast in their own metabolic stew and the temperature levels off ù luckily, long before your leftover hot dogs explode.