It’s a purely academic exercise, says the mapmaker, Elliot Carter — tunnels are cool, and there’s fascination in knowing where the abandoned ones are located.

“Tunnels have kind of become my thing,” Carter said. “I thought it was the neatest thing that all the buildings are knit together in this way that not everybody sees.”

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But he’s facing a challenge: trying to determine where all the tunnels are actually located. It’s not as simple as asking public officials. At a time when government agencies are more aware than ever about the security and terrorism risks that can come with publicizing information about decades-old infrastructure, several have told him that such information is secret and should remain so.

Carter, 27, has a day job in public relations that’s far removed from the wonky world of historical infrastructure. But six months ago, he embarked on his tunnel project, and since then, he says, he’s spent most of his waking time outside of work compiling archival information, visiting libraries, scouring online historical databases and making records requests from government offices.

“I’ve been working on this kind of maniacally,” he said. “If you pick a small enough pond, it’s not hard to know more than almost anybody else. I feel like I’m the biggest fish in this tiny, tiny pond.”

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And so far, he’s accumulated a significant collection of tunnel expertise. The research he’s done so far is displayed on a website, WashingtonTunnels.com, with interactive maps and black-and-white photos and an extensive glossary of sources. Of course, he’s mapped out the easily identifiable tunnels, like Metro, the freight rail tubes, the highway underpasses and the well-trodden walkways in the basement of the U.S. Capitol.

But he’s also uncovered underground thoroughfares that are far less known among average residents. For example, there’s the Washington aqueduct system that starts in the Potomac River at Great Falls, where, as Carter writes, “giant brick drinking straws poke out into the gushing flows.”

Or, there are the steam tunnels beneath downtown federal buildings, which are operated by the General Services Administration. Those tunnels and their associated heating plant played a major role in downtown geography. “The elevated Whitehurst Freeway, for example, is a relic of the time when B&O railroads ran down K Street to deliver mountains of anthracite coal to the hungry government furnaces,” Carter writes.

There are the ivy-covered sand silos alongside McMillan Reservoir near Howard University, which were part of a now-defunct underground sand filtration facility where, Carter describes, “drinking water slowly percolated through cavernous, 30-acre underground sand vaults like a giant municipal Brita filter.”

And Carter’s favorite piece of archaic subterranean infrastructure is the “book conveyor tunnel,” which once ran between the Capitol and the Library of Congress, where stacks of books and reams of documents were ferried back and forth to and from members of Congress.

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“It’s this steampunk gem,” he said.

But there’s one problem. In the case of the book tunnel, it’s proved nearly impossible to get any public official to tell him with certainty whether any part of the tunnel still exists (part of it was destroyed in the construction of the underground U.S. Capitol Visitor Center), whether it remains accessible to maintenance workers, and where exactly it’s located.

He said he struck the same roadblock for other defunct tunnel systems in the region, with agencies citing security concerns that they say prevent them from releasing the historical blueprints.

Carter said he’s not trying to encourage people to go looking for the tunnels or trying to find secret means of accessing them. (There’s a disclaimer on his website: “Exploring tunnels in person can be hazardous and illegal, don’t try it.”)

And the irony is that a lot of this information is already available in sources like newspaper articles and bulletins from the time period when the tunnels were constructed, just as commuters now are kept abreast of the minutiae of road or transit construction projects.

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“It’s all from public records — it’s no more secretive than the Silver Line,” Carter said.

But, he said, knowing the ways in which this physical infrastructure has been repurposed over time, and in some cases completely abandoned, provides insight on the changing appearance and civic inclination of Washington.

“When we think of tunnels now, we think of these super-expensive capital infrastructure projects, and you have committees and boards going back and forth for a very long time about ‘Do we tear up this street or that street?’ It’s all very studied and planned and justified,” Carter said.

A century ago, things were different.

“For a lot of these, there’s no rhyme or reason; it’s clearly just one administrator, like, ‘Here’s what we’re gonna do,’ ” he said.

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The challenge of retrieving this kind of archival information from public agencies is a common problem among academics who try to document the evolution of a city.

Kate Ascher, a professor of urban development at Columbia University, ran into this challenge as she was researching her 2005 book, “The Works: Anatomy of a City,” about the invisible infrastructure that operates New York — little-known features like the pneumatic tubes running underground that were once used to deliver mail throughout Manhattan.

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Ascher said that in some ways, modern technology has made it much easier to obtain hyper-detailed records of the history of buildings. But modern-day sensibilities about safety and security and ever-increasing concerns about terrorism have made it much more difficult to get many forms of historical records.

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“Operations are sacrosanct,” Ascher said, “so anything that could potentially threaten the way things work — they never like to release that stuff, which is kind of understandable.”

And then there’s another much simpler issue: the problem of forgetting. When tunnels are sealed and cease to be used for any critical purpose, there’s a risk that the infrastructure and its insightful history might be forgotten.