What’s That Thing is Slate’s column examining mysterious or overlooked objects in our visual landscape. To submit suggestions and pics for future columns, drop us an email.

The Massachusetts Turnpike is the Bay State’s most famous road. James Taylor got all teary about it. So did the Missouri group The Get Up Kids (“Last night on the Mass Pike/ I fell in love with you”). Good Will Hunting closes with a shot of the turnpike.

On this fabled portion of I-90 lurks something of a mystery: The sign above, located in Becket, Massachusetts, states that in a westbound direction on I-90, the “next highest elevation” doesn’t come until Oacoma, South Dakota.

The sign is regularly photographed and blogged by road-trippers, and remembered with fondness as a familiar milestone. People film the sign, even while they’re driving. John McPhee gently mocked this marker of the “cordillera of Massachusetts” in a 2005 New Yorker story. There’s even a poem about it.

I’m a fan of entirely unnecessary road signs that suggest the scale of America and its highways, and I love this Massachusetts sign even more than the I-40 sign in Barstow, California, that announces the remaining 2,554 transcontinental miles to North Carolina.

The turnpike’s sign serves its poetic purpose—to suggest that you’re about to traverse a large, mostly flat part of an enormous country.

But what exactly do the numbers and the curious wording on it mean? Is the highway elevation of 1,724 feet in Becket not exceeded until Oacoma by a hill that tops out at 1,729 feet before dropping away? Or does the sign mean only that the highway passes through Oacoma, which—somewhere in its borders—has a highest elevation of 1,729 feet?

The sign could also conceivably refer to each state’s I-90 summit. Or maybe there’s some interstate dispute about which spheroidal model of the Earth to use?

I asked two mathematicians, including a topologist, for insight. One described the sign’s wording as “a little meaningless.” The other said “one is left feeling that the sign has subtracted information, not added it.” (By the way, the sign in Becket that eastbound motorists encounter isn’t clearer. It says that the highway’s “last highest elevation” on I-90 was in Oacoma.)

A representative for the Massachusetts Department of Transportation told me: “The sign indicates that in that Becket location, I-90 is at its second highest elevation of 1,724 feet. The highest elevation along I-90 is in South Dakota, at 1,729 feet (5 feet higher).”

Mystery solved. Except, after cutting across South Dakota and a bit of Wyoming, I-90 spends a lot of time in Montana, a state whose name aptly comes from the Spanish word for mountains, with a minimum elevation of … 1,804 feet. The highway passes through Butte, Montana—official elevation: 5,538 feet.

Next I tracked down former Berkshire Eagle editor Don MacGillis of Pittsfield, Massachusetts. MacGillis wasn’t responsible for the sign’s wording, but he was responsible for the sign itself. A former sign had simply marked the location of the turnpike’s highest point, sans mention of anywhere else down the road. In the late 1980s or early 1990s, MacGillis suggested that the Massachusetts Turnpike Authority upgrade the sign to reflect what he knew about American geography—that this very long road probably didn’t exceed its elevation in the skyscraping Alps of Massachusetts until it was past the Mississippi River, more than halfway to the Pacific.

He suspects that the sign’s designers came up with the ambiguous wording by simply checking the highest points on I-90 in all the towns that the highway passed, until they got to Oacoma, where the town’s high point on I-90 finally exceeded the Becket elevation of the road.

But does anywhere in the town of Oacoma—on or off the highway—actually reach an elevation of 1,729 feet? Unlikely, says Derric Iles, the helpful state geologist of South Dakota and the director of the state’s Geological Survey Program. In a phone call, he cautioned that town boundaries can shift but that at first glance there’s nowhere within the current borders of Oacoma that reaches even 1,600 feet, let alone the lofty heights of 1,729 feet. Cheryl O’Brien, a specialist at the United States Geological Survey, couldn’t immediately find any such elevations either, until well west of the town. And in a searchable database of American place names, none in Oacoma had an elevation over 1,600 feet.

Another helpful USGS employee, Steve Reiter, calculated the highest elevation in the municipal boundary of Oacoma as between 1,430 and 1,440 feet. He suspects that the Massachusetts sign mentions Oacoma because it’s the nearest named geographic feature.

But if the 1,729-feet point isn’t even in Oacoma, then that only raises the question of where that number came from and why it’s used on the sign, which might more simply have stated “this elevation not exceeded on I-90 until South Dakota,” or “no higher elevation on this road until you climb the hills west of the Missouri River.”

The mystery endures. Perhaps it will one day inspire a song about the long American miles between two of the continent’s lesser peaks. James Taylor, over to you.

See something out there that you’re wondering about? Send a description and a pic to whatisthat@markvr.com.

*Correction, Aug. 18, 2014: A caption in this post originally misidentified a topographical map of Becket, Massachusetts, as showing Oacoma, South Dakota.

