An original copy of John Randel, Jr.’s Manhattan grid map at the County Clerk's Office, in New York. The intersection in focus, Sixth Avenue at West Eighty-sixth Street, is now a part of Central Park. Photograph by Michael Appleton / The New York Times / Redux

On an overcast day in November, 2014, just before Thanksgiving, two men dug a rather large hole in a lawn in Central Park. They started at seven-thirty in the morning, and by midday the hole was big enough for them both to stand in. As they dug, they filtered excavated soil through a screen. They found eighteenth- and nineteenth-century Chinese porcelain, blue earthenware fragments, and the rim of a pearlware teacup, as well as the stem of a clay pipe and brown, olive, aqua, and purple glass shards. And they unearthed a roughly three-foot-tall, nine-inch-square white stone, two sides of which were inscribed with numbers.

The ceramic and glass remnants were unexceptional, but the white stone was anything but. It was a discovery akin to finding a marble statue submerged in a remote lake or a lamppost in the wild woods of Narnia. Hundreds of stones like this one were fastidiously implanted across the island two centuries ago, but not a single one seemed to have survived, in its original position, amid the construction and endless reconstruction of New York City. The stones were set at the intersection of every street and avenue to chart the bold nineteenth-century plan that gave Manhattan its great grid. The carved marble sign in Central Park marks an intersection that never came to be, one of many spliced out of the grand plan when city residents demanded an antidote to the grid.

Central Park has long kept its grid memory secret. But in little more than a year since that November morning, three more marble street monuments have been discovered in the curvaceous green core of the island.

A map of the City of New York, from 1811. Photograph by The New-York Historical Society / Getty Photograph by The New-York Historical Society / Getty

In 1807 the Common Council asked the state to appoint three commissioners to plan the city’s development. (The aldermen were hoping to avoid the disagreements and political reversals that occurred at the local level; and they did maintain some say by recommending three men who should serve as the commissioners.) They hired a young Albany native named John Randel, Jr., to survey the island and draft the 1811 Commissioners’ Plan, an eight-foot-long blueprint for the grid, which was to run from North (now Houston) Street to 155th Street. After Randel handed it in, the Common Council hired the exacting surveyor to inscribe the grid in the rural landscape. Randel resurveyed the island with instruments of his own invention, placing wooden stakes or pegs at every one of the more than fifteen hundred planned intersections. Once done with that task, he and the bane of his meticulous existence—his unruly, ever-shifting, drink-loving crew—set about replacing the pegs with less easily vandalized or purloined markers. At some fifteen hundred and fifty intersections, according to Randel’s notes, the men set “monumental stones”; at nearly a hundred others, where they encountered bedrock or boulder, they placed iron bolts.

The process took until 1817, with the men carrying or carting monuments to intersections, often working up one avenue and then down another. Sometimes they tried to avoid lugging the heavy stones: “This delay has much the appearance of a determination to get here too late to carry monuments,” Randel wrote in one of his field books. He, too, found the monuments, which were much more expensive to buy, carve, and install than he had anticipated, to be a burden. And although the hefty, largely interred stones were challenging to remove, landowners still occasionally uprooted them.

As the city extended up the island, the monuments and bolts seem to have become rubble, dug up and discarded as the terrain was transformed. The sole documented marble monument, for more than a century, from a corner of Fourth Avenue and Twenty-sixth Street, survives in the collection of the New-York Historical Society. Construction workers unearthed it in 1890 and it was, for a time, on view in Madison Square Garden. Not too long ago, the Historical Society discovered a second monument in its collection, from Second Avenue and Ninety-second Street. For many years it lay partially buried in the Society’s garden. It was excavated in 2006, mistakenly catalogued as a milestone, and transferred to a warehouse in New Jersey.

One well-known bolt endures, embedded in a low rise of gray schist in the park’s southern section. It was discovered and confirmed to be in the right location, in 2004, by Reuben Rose-Redwood, a geographer at the University of Victoria, and Lemuel Morrison, then of Mercator Land Surveying, who hunted for Randel bolts throughout Central Park when Rose-Redwood was conducting research on the 1811 plan for his master’s thesis. “We never looked for monuments,” Morrison said. “We just presumed they were all gone.” As have most people who search for, or are intrigued by, survey markers and a landscape view of New York City history.

The archeologists who excavated the monumental stone that November morning—James Lee of Hunter Research, a Trenton, New Jersey-based company, and Matthew Pihokker, formerly of the same—had been hired the year before by the Central Park Conservancy to study the northern stretch of Central Park following the restoration of the six-acre Fort Landscape, rocky heights where Fort Fish, Fort Clinton, and Nutter’s Battery once stood. They excavated a gatehouse at McGowan’s Pass and found evidence of other defenses hastily built during the War of 1812. They uncovered traces of the Kingsbridge Road, which wound north up the island on its way toward Albany and Boston.

One spring day during that project, the president and principal archeologist of the company, Richard Hunter, was strolling to meet his team in the field when he saw a flat white stone embedded in a lawn. The team had with them printouts of several of Randel’s farm maps—a set of ninety-two beautiful, topographically accurate, large-scale maps that the surveyor drafted for the city between 1818 and 1820—which they frequently consulted. One showed the notation “mon,” Randel’s shorthand for marble monument, seemingly near where the flat white stone sat. Although intrigued, the crew was focussed on the Fort Landscape, not on the grid, and returned to their investigation of the northern park.

And perhaps the flat white stone Hunter saw that May day would have remained unrevealed, were it not for Richard Garland, a retired software engineer and history maven who regularly runs and walks in Central Park, scanning the landscape with the hawk eyes of a surveyor. After hearing a talk at the Museum of the City of New York, in September, 2014, about the archeological finds in the Fort Landscape, Garland e-mailed Hunter asking if the crew had seen anything resembling a bolt or monument. Hunter recalled the flat white stone, although in his memory it was two to three inches on a side.

Garland quickly visited the stone. It seemed to be eight-and-a-half inches to a side, which he identified as the dimension of the monument in the Historical Society’s collection. Garland contacted Morrison—the two had sought Randel survey markers before, in Highbridge Park and Riverside Park—and Morrison toted his G.P.S. equipment to the spot, took readings, and checked them against maps in his office. The stone’s coördinates were within one foot of the expected position: a match for a Randel marker. Garland then catalyzed the official quest by letting Hunter know about Morrison’s measures. Hunter contacted the Central Park Conservancy. A month later, the archeological excavation revealed the flat white stone to be the city’s only known in situ grid monument.