At dawn, squatting by a railroad track near the Napa County Airport, Victoria Langenheim unloads her gravity meter and the stout, featureless twelve-volt battery pack wired to it, nicknamed “Champ.” Champ weighs ten pounds and replaces “El Primo,” Langenheim’s last battery. “It wasn’t so el primo after five or six years,” she says.

The gravimeter itself has no nickname, just a number: G614. It costs us$50,000 and is treated like a prima donna wherever it goes. It is a heavy-looking box, 15 inches tall, and resembles a ramshackle stereo component with dials, gauges and an LED screen embedded on its top face. Post 9/11, it is no joy to carry through airport security.

A periscope peeks into the meter’s hollow cavity where a tiny light bulb illuminates a one-ounce weight dangling on a slender spring. The spring, made from a proprietary metal alloy, is insufferably sensitive. It abhors heat and shock; even flicking on the meter’s own light upsets it. When making measurements in the Mojave Desert, Langenheim must stand over the gravimeter with a parasol or attend to it with a towel moistened with her own drinking water. To protect the spring, the gravimeter is conveyed in a hulking metallic briefcase and must be seat-belted like an infant on a cardboard platform in the backseat of Langenheim’s government-plated Suburban. When taking roadside measurements, the spring fidgets if a tractor-trailer passes within several hundred yards and could snap in two if not clamped immediately. The previous week, a team from Langenheim’s department had to lock down their gravimeter and cease work for two hours when an aftershock of the Sumatran earthquake sent it jigging ferociously. This was a full seven months after the quake; they were in Nevada.

The United States Geological Survey’s Geophysical Unit of Menlo Park, California (GUMP) owns six gravimeters. Each is numbered and exhibits a personality over time. Langenheim has selected G614, her favourite, for this trip, though she also has a good rapport with 17C. Another, G8, is notoriously finicky, particularly since falling off a horse. It also dropped off a boat in Alaska. Another, G17, plummeted down a crevasse in Antarctica; a mountaineer was sent in after it to salvage the parts. (Gravity, it turns out, is the gravity meter’s worst enemy.)

Langenheim and GUMP’s other potential-field geologists are, in effect, gravity hunters, mandated by the American Taxpayer to measure fluctuations in the earth’s gravitational field. They travel around detecting tiny variations in the downward pull on the meter’s weight—the weight of the weight, essentially. They can peg gravity down to the microgal, or one-thousandth of a centimetre per second-squared. (This is nine decimal places past the 9.8 metres per second-squared at which we generally assume gravity to accelerate falling objects—if we think about gravity at all.) Gravity varies slightly from spot to spot relative to the density of the earth there; areas of denser rock have stronger gravity. Mapping a region’s gravity is thus a form of X-raying. Given the gravimetrics of a region, a skilled geologist can interpret what’s happening underground. A pattern of weaker gravity, for example, might outline porous, lighter rock through which groundwater might be flowing. In the early nineties, Langenheim used gravity to map buried volcanoes near Yucca Mountain in Nevada, seeing how safe the US government’s proposed nuclear repository there would be.

A single gravity station, or measurement point, tells you virtually nothing by itself though, and the minute-by-minute business of “making gravity stations,” as it’s called, is accordingly dull. Langenheim’s first measurement of the day is representative and goes something like this: Craig McCabe, her burly young assistant, jabs at his handheld GPS computer with its stylus, so they know precisely where on Earth they are measuring. Langenheim crouches, puts the meter on its base plate and unclamps the spring. McCabe calls out their location and elevation. She tells him the time. His GPS chirps. She squints through the periscope and, for many seconds, nudges a dial to calibrate the spring. They wait. After several minutes the spring steadies, and Langenheim calls out the reading, 3528543, which they each record. “You’re basically just weighing the earth in different spots,” Langenheim tells me. She is unable or uninterested in making it any more glamorous than it is.

Langenheim—a wiry, scrappy-looking woman in sunglasses, drawstring pants and a dusty, oversized T-shirt—estimates she’s gone through this exact exercise about 7,500 times at as many different points on Earth since 1985. (Her mother was a paleontologist at GUMP, and Langenheim started there as a typist after high school.) She’s made measurements across California, the Northwest, Southwest, Gulf Coast, Antarctica, Italy and Puerto Rico, on all sorts of terrain. She’s schlepped her meter through the ghettos of South Central Los Angeles and made gravity stations at half-mile increments up and down the Las Vegas Strip.

Today’s fieldwork is part of Langenheim’s two-year project to map the spindly network of faults between Napa Valley and Santa Rosa, indicated by trails of abrupt changes in gravity. The government funds projects it deems to have high public interest. In this case, as people move north out of Silicon Valley and sink their savings into new homes and vineyards, planners want to know where the region’s faults and groundwater may lay. But the main reason Langenheim is measuring gravity here, it seems, is that no one has yet.

Langenheim has made 1,000 measurements over two years and has only about 50 more to fill in. This, she hopes, will be her last trip for the project, she explains, spreading a map across McCabe’s torso to show me. The map is speckled with thousands of multi-coloured, stick-on dots. A swarm of yellow ones in San Pablo Bay are gravity stations made by Chevron in the sixties and seventies. (Since the thirties, oil companies have used gravity as an enlightened form of dowsing—a cheap, clean way to feel out petroleum deposits without having to dig.) But like most oil company data, the Chevron measurements remain proprietary. This is unfortunate, since they cover a marshland Langenheim would rather not have to trudge through. She’s decided to just let them be.

The magenta dots are individual gravity stations Langenheim has made herself, though brokering access to the last remaining ones needed to complete her map has been increasingly difficult. Most sit on private property, behind gates, throughout a posh patchwork of vineyards and rolling estates.

“All the easy stuff has been done,” she says. “Two years ago, we were getting fifty or sixty gravity stations a day.” Yesterday they surprised themselves by netting thirty-three. The day before, McCabe’s first, they had to settle for ten. The morning’s next five stations take us onto the grounds of a water-treatment facility.

Later, a few miles up a heavily wooded road, the scientists leap out of their Suburban and, almost in unison, fasten their broad-brimmed nylon hats. McCabe begins pacing with his GPS computer outstretched, looking for a signal, while Langenheim scouts around. They’ve arrived, unwittingly, in the loose-gravel parking area of a hilltop home. Crooked purple artichoke plants tower in the garden near constellations of Japanese lanterns hung from trees. Below, little worn Toyotas perambulate through the stepped alleys of vineyards, and miles beyond that, a lake blots out the horizon. “This is a good spot,” she says.

Almost immediately, the homeowner’s personal assistant scoots up the driveway in a black BMW and announces herself. She leans backward, as Langenheim asks for permission to investigate the driveway’s gravity.

“And what is the purpose of this?” the woman asks. Langenheim later tells me that biologists tracking endangered species have posed as geologists in order to gain access to properties in affluent areas like Sonoma and Napa Valley. They are not often welcome, since finding such a species limits the landowner’s ability to develop.

“It’s in support of geologic mapping studies,” Langenheim begins, but her explanation ultimately baffles the assistant who prefers simply to collect their USGS IDs and retreats inside.

Eventually, a heavy-set man in a polo shirt embroidered with the name of a local winery runs as best he can from around the side of the house. Langenheim starts over. The man is jocular and couldn’t care less. Handing back their cards, he tells her, “You know what? Go for it. Anywhere you want to find some gravity, go ahead.”

For a moment, measuring gravity feels festive. Then Langenheim turns back to her meter, unloads it, hunches, squints and waits.

The earth is not a perfect sphere. Our planet, at the surface, is mostly liquid, after all, and its rotation warps the globe into something more scraggy: wider at the equator than at the poles, rife with bulges and depressions. Local gravity measurements can illustrate this erratic lumpiness. The more gravity stations scientists collect, the more accurate figure of the planet they can draw. This painstakingly precise model of the planet, warts and all, is called the geoid, and it resembles a Play-Doh ball fashioned by an uninterested six-year-old.

It’s convenient, and more flattering, to imagine the world as perfect, and we almost always do, employing a uniform, beautiful sphere called the ellipsoid. For centuries, humankind was content to go on fudging it this way. But, at any given point on earth, there is usually a gap between the imaginary, smooth surface of the ellipsoid and the actual, irregular geoid. Though map-makers were aware of this incongruity as early as the eighteenth century, there was no real incentive to deal with it until the Second World War, when armies began figuring out how best to fling guided missiles across the earth.

Without correcting for the gap between imagined ellipsoid and actual geoid at a launch site, it’s hard to know at what angle you are actually firing a missile. The flat, smooth surface you assume you’re standing on might actually be bumpy and tilted, and what looks to you to be straight up (perpendicular to the earth) might actually crook off slightly in some other direction. What’s worse, any discrepancy at launch gets exacerbated the further the missile travels—a rocket fired at an unexpectedly skewed angle from California to Russia will continue travelling on this different course all the way across the Pacific, possibly ending up miles from its target. Similarly, not accounting for gravity anomalies at the target means putting the missile down somewhere slightly different than where you intended.

When it comes to steering nuclear warheads, this imprecision could be disastrous. Yet the US military’s attitude toward the problem long boiled down to a kind of Zen koan: as historian Deborah Jean Warner put it to me, “If you don’t know that your missile is going to land within a hundred miles of your target anyway, does it matter if you know exactly where downtown Moscow is?”

Plainspoken and spirited, Warner is a curator at the National Museum of American History, a division of the Smithsonian in Washington DC. As World War II and Cold War documents have been declassified over the years, Warner has reconstructed the once-secret history of American military geodesy.

America’s most “precise” guided missiles in the nineteen-fifties, like the Navajo and Snark, were considered successful with their 50 percent chance of striking within 3,000 feet of their target. But the military understood that the new breed of more dependable, less clumsy missiles required a correspondingly accurate approximation of the earth. They’d also need a contiguous one; you could have the most detailed map of California and the most detailed map of the Soviet Union spread out on opposite sides of your table, but without knowing what’s in between those two places, you’d be hard pressed to get a rocket from one to the other with any accuracy.

The military needed a master geoid, cobbled from gravity, satellite and other measurements across many different points on earth. This project got a jumpstart when, in March 1945, a US Army officer named Floyd W. Hough led his top-secret Hough Team into Germany, capturing a cache of geodetic data from the Germans (including Soviet data the Germans had swiped). Soon Allied scientists were integrating the whole mess into a grand European datum. For this, Warner dubs Hough “a geodetic hero.”

The American military would soon send teams out across the globe to squat over their gravimeters and bring back the numbers, but wrenching funds out of Congress for these projects proved difficult initially. Geodesy was science, after all, and a particularly abstruse and un-macho one at that. “We need a new battle tank—okay,” Warner explained. “We need these delicate and recondite instruments, and we need to train people and send them all over the world, and maybe, from this obscure science, we’ll get a better picture of the size and shape of the world…? That’s harder for Congress to understand.”

Gravity?” Langenheim quipped when I asked her about the stature of her field today. “Yeah, it’s the law. Big deal. Earthquakes get a lot of attention. Or if you study volcanoes…It’s like, Wow! Volcanoes! Cool, dangerous. People can die.”

She was making a measurement behind their truck while McCabe, having nursed a large Starbucks coffee all morning, used a nearby porta-potty. This was an unexpected luxury; normally, they just set off into the scrub with a shovel they keep in the back of the Suburban.

Langenheim is a civilian, doing civilian work. But her gravity data, like all data collected by the geologists in her department, are also shared with the National Geospatial–Intelligence Agency (NGA), the US military’s current, very well funded department of geodesy and mapping. Along with data NGA acquires from oil companies and private contractors, the gravity stations Langenheim was making today would be absorbed into the agency’s all-encompassing, ongoing refinement of the geoid. At the same time, in co-operation with colleagues across the country, Langenheim is building an online, open-source civilian encyclopaedia of gravity—trying, as she put it, “to make it easier for people to get gravity and use it.”

In a certain sense, both this and the work by the NGA are part of an illustrious and millennia-old project: humankind getting to know its earth. What’s being built, gradually, in almost geologic time, by a loose association of authorities with decidedly different motives, is the most detailed representation of the planet possible. That venture may have started in earnest around 250 bc when Eratosthenes, one of the librarians at the Great Library of Alexandria, used basic geometry and the angle of the sun in two different cities to calculate the circumference of Earth. Astoundingly, he only overshot by about 5,000 kilometres. It’s difficult to imagine how wondrous and infuriating it was for a simple man in Alexandria to find out his world was far larger than he’d ever be able to explore.

Admittedly, immensity and wonder weren’t on my mind while hanging out with Langenheim. I was almost immediately bored with gravity hunting. When she took the first measurement, I crooked excitedly over the meter next to her. By the day’s fifth measurement, I was staring vaguely into a nearby thicket. I thought a lot about lunch.

“Squatting over a gravity meter is not very sexy,” Langenheim went on as she hunched over the box, waiting for the little, temperamental spring to humour her with a reading. “I have a couple of pictures of me taking a gravity measurement on my website, and I’m like, Wow, that really makes your butt look big.”

After darting around deserts or the North Slope of Alaska weighing the earth, GUMP’s potential-field geologists like Langenheim always return to what is, of course, just another office. Until a massive round of layoffs about ten years ago, they had a softball team that competed in an inter-office league against, presumably, more pedestrian outfits like law firms and communications consultants. GUMP’s team was called “the Potential Fielders,” which was amusing, Langenheim says, because they weren’t very good.

The division has its office politics too, especially since the work often requires teaming up for long trips to lonely places. Langenheim described her former field partners with a warmth, or bitterness, that mirrored her discussion of the various gravimeters’ personalities. Her first co-worker was a delightful guy from Denver who, being six-and-a-half feet tall and long-legged, was difficult for her to keep up with in the desert. He also introduced her to beer. Her last co-worker, before McCabe, was a young woman with a “radiating resentment” and a blood-sugar problem who once survived on blueberries for a week and then wondered why she got ill.

Much to my disappointment, the National Geospatial–Intelligence Agency—the military nerve-centre of this grand, ever-growing network of data—is, it seems, just an office too. Deborah Jean Warner visited the agency’s facility in St. Louis while conducting her research. When I asked her to describe it to me, she shot back, “What was it like? The guy at the front desk talked to me about football.”

NGA has more than 50 million individual gravity stations on record. But there is no magisterial, master archive room of gravity. If there were, its site could be located, targeted and destroyed. “Literally, it’s the age of distributed processing,” the spokesman said. It’s digitized and redundant, of course, all in the ether. “We have no central location by design.” This is to say, the world’s most exhaustive library of the world’s most scrupulous global positioning information exists in no one place.

Nothing can remain immense if it can be measured,” Hanna Arendt wrote in 1958, just about the time when American military scientists were preparing “Geodesy for the Laymen”—a little pamphlet distributed to every member of Congress to explain the importance of funding geodetic mapping. Yet after my loping travels with Langenheim, I found the very fact that gravity can be measured—that there is something to measure and that it’s actively being measured with stupefying monotony—made the world seem even more immense. This tedium, paradoxically, made humankind’s multi-faceted mapping of the earth a spectacularly disarming feat. Immensity is being manufactured one grubbing gravity station at a time. In the long hours I tagged along—for all Langenheim’s squatting, waiting and moving on to the next station—her map only gained a handful of magenta dots.

That afternoon in Napa, another long, uphill road led us to a small winery and olive-oil mill. We parked beside a red farmhouse with a battery of twenty-foot stainless steel vats in the back. There was a crucifix over the door and, when Langenheim rang, a white-haired woman in a blue shirt with many pastel fish batiked on it, answered. She was smiling, as though she were expecting us.

As Langenheim started in about her study, the woman nodded, rocking her head ever so slightly forward and back as though she were listening to music. Then, in a tickled tone of voice that was not quite a question, she said the word herself: “Gravity.”

“It changes,” Langenheim told her, by way of explanation. Clearly, this news surprised and impressed the woman greatly. “Oh yeah,” she would say, slowly, from time to time, as Langenheim kept talking. Her eyes were wide and she said, again, “Gravity”—as though she’d never really thought about gravity before and had now discovered a great pleasure in doing so. She assented to the measurement, very gladly, and asked to be mailed a copy of the completed survey.

The measurement was made in the centre of her driveway. While they waited for the spring to steady, a pheasant walked by leading a small cluster of chicks. Langenheim, meanwhile, was telling me about a squalor-filled gravity-measuring excursion through a southern California town called Azusa. It was a bleak place, and around where they stayed the only thing to eat was fast food. “We might have just hit the wrong hotel,” she said, trying to be fair. Then she mashed her right eye into the gravimeter’s periscope and read off another long number.