What is the deepest cave in the world, and can you go into it?

—Ken Severson, Seattle, Washington

Think there are no untrodden places left on earth? Geologists estimate that fewer than half of the planet's cave systems have been located, let alone surveyed, and that the deepest cave—measured in vertical distance from opening to end—is yet to be found. The current record-holder is Lamprechtsofen-Vogelschacht, a limestone chasm that ends 5,354 feet below its opening in the northern Austrian Alps. Though the cave was first discovered five centuries ago, it wasn't until August 1998 that Andrzej Ciszewski, a veteran Polish caver, finally reached its end. (This beat the previous record, obtained in a cave system in northern France called Gouffre Mirolda-Lucien Bouclier, by 74 feet.) But don't expect L-V to remain the champ for long. Experts suspect that Oaxaca, Mexico's Sistema Cheve could be deeper still, judging from a test in which traceable dyes were added to the cave's water system and found later in a distant canyon 8,000 feet below the opening. Before you rush out to probe the world's murky bowels, though, take heed: They require El Cap–caliber climbing skills (and twice as much equipment) to negotiate sheer overhangs, waterfalls, and narrow passageways—in the dark. Explorers also need scuba expertise to get through vast underground lakes called "terminal sumps." "These caves aren't exactly places you'd want to recreate," warns Ron Kerbo, cave management coordinator for the National Park Service. "Going into them is more like an expedition to Everest."

Why is it that when you stop paddling a kayak, it turns sharply instead of maintaining a straight course?

—Addis Holmstrom, Edina, Minnesota

Imagine a veering kayak as a kind of waterborne, featherless dart. It has very little resistance in the rear, which makes it highly susceptible to wobbly commotion. As a kayak moves through roiling whitewater, or even across a glassy lake, pressure builds up on the bow from water pushing against it. Simultaneously, small whirlpools called eddies form on both sides of the boat near the stern, which surge and press against the boat on each side. But here's the catch: Not all eddies are equal, and the strongest (typically on the side opposite your most recent stroke) will push the stern away from it, forcing the boat's rear to veer off course, thereby causing the bow to swing in the opposite direction. The imbalance is corrected each time you paddle forward, but it overcomes your rig when you coast. And while this yaw may cause a kayaker to feel off-kilter, a boat designed to turn less easily would be more suited to carrying freight than negotiating Class V rapids or rough seas.

How did something so obviously disadvantageous as poor human eyesight survive the evolutionary process?

—Guy Silker, New York, New York

A variety of factors, including heredity, determine an individual's eyesight. First, 20/20 vision is hardly "normal," given that less than 50 percent of the general population sees with such natural acuity. Trying to account for a recent surge in American nearsightedness—which afflicts almost 45 percent, according to one count, up from 20 percent decades ago—experts now believe that the eye can change its shape in just a few years. After prolonged, close-range work, the curved cornea—a lens that reflects images onto the retina—can actually grow steeper, a shape that is optimal for reading, but causes objects at a distance to appear blurry.Environmental factors aside, some poor eyesight is inherited, a fact that begs an evolutionary explanation. In effect, offspring with poor eyesight were protected by those with better vision long enough to procreate and pass their weak eyes on to generations of squinting progeny. "We humans nurture our offspring almost to the point of reproductive maturity," explains Joram Piatigorsky, chief of molecular and developmental biology at the National Eye Institute in Bethesda, Maryland. "Which means that poor eyesight or perfect eyesight, the trait gets passed on"—as it was passed from the earliest primates to our cousins the gorilla and the chimpanzee, who care also for their young until almost child-bearing age, and who have also been known to suffer occasional fuzzy vision.

How do birds find worms that are underground?

—Adrienne Tsou, San Diego, California

Simply put, hungry birds hear their prey eating. "The worms aren't exactly humming show tunes," says Patrick Weatherhead, a biologist at Ottawa's Carleton University. "But their gustatory sounds are enough to give them away." Worms feed by passing soil through their bodies, a process that makes a low-frequency sound not unlike the faint crunching of footsteps over gravel. Using its thumbtack-size ears, a hungry songbird can detect the sound from up to five inches away (hence the common sight of robins hopping along the turf with their heads cocked toward the ground). The bird then flares its tiny nostrils to scent out the eau de worm. Having pinpointed the exact location, the bird goes in for the kill with an elaborate maneuver, kicking its legs out from under its wings and jackhammering its beak a few centimeters into the soil to nab its wriggling breakfast.   

Illustrations: Jason Schneider

Send your questions for The Wild File to Outside, 400 Market St., Santa Fe, NM 87501, or submit them here.