These are the buildings that make up the 'cloud'

Everything you know about the Internet is wrong. Virtual? Try physical. “The cloud”? Try the ground. Our beloved information superhighway has very real exits. Behold the “carrier hotel.”

You’ve got to admit, the concept of “the cloud” is one heck of a marketing coup. Somehow technology companies got the world to think that the Internet is as ubiquitous and fleeting as patterns of water vapor in the sky. But the Internet is far from gossamer—it’s a jumble of copper cables and server stacks wrapped in the windowless walls of hulking urban structures. There are indeed interchanges on our infobahn, and they’re called Internet exchange points, or IXPs—or colloquially, “carrier hotels.” They are ultra-secure buildings that serve as nodes on a network that spans continents. They help connect Internet service providers to one another. They are the real “cloud.” Here’s a look at six of the world’s largest.

—Andrew Nusca

One Wilshire Boulevard

WEST COAST: LOS ANGELES

Sandwiched between bourbon bars and boutique hotels, this unassuming high-rise in downtown L.A. is a hotspot for telecom firms. Why? Because it’s where fiber-optic cables from Asia and North America meet.

Towering. Massive. Immense. In a city as low and sprawling as Los Angeles, you can’t miss the skyscrapers that stud the city’s bustling business district, especially as you drive along the freeway toward the city’s downtown district. It’s easy to assume that these tall buildings house Hollywood executives or California bankers. Many do. But one of them, a 30-story structure called One Wilshire, is the most highly connected Internet point in the western U.S.

Miles of undersea cables link the Pacific Rim to One Wilshire. It is estimated that one-third of Internet traffic from the U.S. to Asia passes through the building. For this reason scores of service providers, online content distributors, and carriers call One Wilshire home. Verizon VZ -0.21% , AT&T T -0.23% , Amazon Web Services AMZN 0.47% , and Netflix NFLX 4.19% have all set up shop in the structure. Its biggest tenant is CoreSite COR -0.90% , a national data center and collocation provider, which leases its space to more than 300 companies.

At the heart of the building is the Meet Me Room, a sort of neutral ground for the building’s tenants to interconnect. It’s in this room, which contains rows upon rows of cabinets like lockers lining a high school hallway, that CoreSite’s data-center operators manage the flow of traffic throughout the facility. If one of the companies in the building wants to connect with another, CoreSite staff in the Meet Me Room ensure that a handshake takes place and traffic gets swapped. If One Wilshire is a carrier hotel, the Meet Me Room is its lobby.

One Wilshire was erected in 1966 as a standard office building. The 664,000-square-foot structure served the role for many years, but as traditional corporate tenants moved out in the early 1990s, telecommunications companies moved in, attracted by its proximity to AT&T’s switching center two blocks down the road. At the time, such infrastructure was nascent, and telecom companies congregated in single locations. As the Internet boomed, so did One Wilshire, and the facility eventually became the de facto connection point for Southern California and beyond. How important is it? Consider: In 2013 it sold for nearly half a billion dollars and the highest price per foot ever paid for a downtown L.A. office building.

Given Los Angeles’ reputation for earthquakes, there remains concern that a natural disaster could strike One Wilshire. If that happens, tenants would funnel Internet traffic to other facilities in the region, as far away as Arizona or Nevada. Down, but not out: The modern Internet lives on to fight another day.

—Jonathan Vanian

60 Hudson Street

EAST COAST: NEW YORK

It was built in the wake of the 1929 stock market crash as one of the most advanced telecommunications hubs in the world. Eighty-five years later it’s making neighbors of nations at the speed of light.

If a building could talk, 60 Hudson Street in lower Manhattan would have decades’ worth of tales to tell. Completed in 1930—the same year that Fortune first went to press—the former Western Union Building towered over its New York City neighborhood. It was built to be a nerve center for the telegraph era. Today it’s a nexus for the Information Age.

Sixty Hudson is one of the world’s most important buildings for Internet connectivity—one of about a dozen, according to Andrew Blum, author of Tubes, a book about the Internet’s infrastructure. It sits on the doorstep of America’s financial and media hub and carries Internet traffic from the rest of the world to North America. Its Art Deco lobby and massive brown-and-red-brick façade, which zig-zags back from the street and up 24 floors, are enduring symbols of its historic role as a first stop for transatlantic cable traffic.

Proximity to the major population centers of the U.S. was a key reason 60 Hudson was built where it stands today, less than a mile from the New York Stock Exchange. In a time of split-second financial transactions, on-demand movie streaming, and anywhere Internet, the building’s coveted location—paradoxically—remains critical. “This building is important more than just to New York,” says Jonathan Hjembo, an analyst for research firm TeleGeography. “It has key interconnections to Europe and to Latin America.”

Today 60 Hudson’s massive clay conduits hold miles of fiber-optic cable, not just copper telegraph wires, says former Telx exec Ben Gonyea. Their tremendous density remains a calling card. When the New York City Landmarks Preservation Commission designated 60 Hudson a historic site in 1992, it estimated that the structure once housed 70 million feet of wire and 30 miles of conduit. That was well before the Internet became a fixture in our lives.

It’s anyone’s guess how much wire or Internet traffic 60 Hudson accommodates today—the information is confidential and difficult to obtain. Here’s what we do know: Telx, one of two major data-center providers in the building, is home to about 350 carriers. The other, Equinix EQIX -1.39% , hosts about 150. So the next time you sell a share or order a package, remember: Your bits are likely flying through the Art Deco portals of 60 Hudson Street.

—Barb Darrow

International Internet Hubs

North America isn’t the only game in town. Here are four key facilities on other continents.

Hong Kong Internet Exchange (HKIX)

ASIA, LOCATED AT THE CHINESE UNIVERSITY OF HONG KONG

The Hong Kong Internet eXchange is different from other exchanges in that its main purpose isn’t to link out to the wider world. (Unsurprising for a city administered by China, though HKIX is spared from its Great Firewall.) Instead, the facility is meant to serve as a booster for the city’s own internal connectivity. In fact, the whole point of the operation—better termed an intranet exchange—is to prevent traffic from having to route internationally (as in through the U.S., which dominates global Internet infrastructure like no other country). Call it a defensive play: While hiding out in the city in 2013, former National Security Agency contractor and whistleblower Edward Snowden told the South China Morning Post that Chinese University, where HKIX is located, was a prime hacking target for the notorious U.S. spy agency: “We hack network backbones—like huge Internet routers, basically—that give us access to the communications of hundreds of thousands of computers without having to hack every single one.” The NSA declined to comment.

Amsterdam Internet exchange (AMS-IX)

EUROPE

Fiber-optic cables in the Amsterdam Internet Exchange (AMS-IX)Photo: Klerkx—Laif/Redux

The Venice of the North has long been an important center for commerce and trade. The same holds today, electronically. Geographically situated between a number of important digital destinations—Frankfurt, London, Paris—the Amsterdam Internet Exchange serves as one of the biggest traffic routers in the world, channeling roughly 700,000 terabytes a month. Deloitte estimates that the digital infrastructure of the Netherlands added up to 19,000 jobs to the Dutch economy in 2013. AMS-IX is not quite the modern digital equivalent of the Dutch East India Co., but it is an incredibly important nexus in the global fabric of the Internet.

DE-CIX Frankfurt

EUROPE

Switches inside the DE-CIX Internet Exchange in Frankfurt.Photo: Michael Danner—Laif/Redux

This German Internet exchange is a well-placed way station in the world’s data-coursing nervous system. According to TeleGeography, DE-CIX Frankfurt—the flagship in a family that includes facilities in New York, Istanbul, and Dubai—is the No. 1 Internet traffic hub on the continent. During peak traffic times, the exchange can move data at a rate equivalent to processing 4 billion emails per second. While Germans reacted with outrage to revelations concerning the extent of the NSA’s surveillance in their country, their indignation was checked when they learned that the BND, a German spy agency, had tapped the wires at DE-CIX.

PTT Metro São Paulo

SOUTH AMERICA

This Brazilian facility is the biggest and most trafficked Internet exchange in all of Latin America. Nearly 600 networks (including Facebook FB 0.42% and Google GOOG 0.33% ) have taken up residence in the exchange—as many as the company’s 24 other facilities combined. Because so many transatlantic submarine cables land ashore along Brazil’s gigantic, meandering coastline, the country has quickly become a telecommunications hotspot. The nation has pushed aggressively to expand its digital infrastructure and rely less on the U.S. and its well-known tech giants, especially after learning that it was the second-most-snooped-upon by the NSA—after the U.S., of course.

—Robert Hackett

A version of this article appears in the June 15, 2015 issue of Fortune magazine with the headline ‘Guardians of the Digital Galaxy.’