One of London's most famous landmarks is an engineering marvel that was once the tallest skyscraper in Britain. Celebrities and royalty spent their evenings there, and thousands visited it, yet for decades it officially did not exist. This is the BT Tower.
If you're of a certain age, as I am, you might remember a time when the BT Tower was called the Post Office Tower, and dinosaurs roamed the earth. From its site in Fitzrovia, it dominated the cityscape like some visitor from the future that had landed in the metropolis. Standing 620 ft (189 m) tall, the huge steel and concrete cylinder couldn't help but catch the eye – especially with the forest of antennas and microwave horns clustering its upper stories.
There was a time not so long ago when simply taking the image at the top of this article or even referring to the subject as the BT or Post Office Tower would technically have been a violation of the Official Secrets Act and a criminal offense. Even though the Tower could be seen from all over London and the surrounding counties, and was a great source of national pride, the government maintained its top-secret status well into the 1990s. Popular legend often claims it was entirely scrubbed from Ordnance Survey maps to leave a blank space, but the reality was even more absurd: the state officially refused to acknowledge the building even while it was printed on standard maps and stared millions of Londoners in the face every day.
It's just one aspect of the story of one of the strangest pieces of modern architecture from the 20th century, and it reflects the rapid pace of modern technology and how we respond to it.
The Tower had its origin in the 1950s when Britain was undergoing a boom in telecommunications. The General Post Office's (GPO) system of wires and cables simply could not keep up with the ever-increasing demand for more bandwidth to accommodate not only phone traffic but the expanding television networks. By the end of the decade, the subterranean copper coaxial cable networks were on the verge of saturation, so some way had to be found to move all that telecommunications traffic in and out of central London.
That is where they hit a snag.
The obvious solution was to lay in new cables with much greater capacity. That's simple enough to put into a sentence (which I just did), but London was a 2,000-year-old city of eight million people. The terrain under the buildings and streets was already honeycombed to capacity with underground train tunnels, subways, sewers, pipes, conduits, power cables, telephone lines, the odd forgotten river, and so much more besides. Putting in a massive new underground cable network to connect the capital with other towns and cities was nearly impossible with the technology of the time.
The answer was found in some electronic lateral thinking. Instead of going down, the GPO engineers decided to go up. Cables were abandoned in favor of high-frequency microwaves that could be beamed to relay stations outside London and, from there, to the rest of the country.
It was feasible, but the linchpin of the new microwave network would have to be a tower smack in the middle of London that needed to be high enough to see the relay stations, clearing all intervening buildings, projected architecture, and radio masts. This was no easy feat when you realize how low and flat London is, sitting in a huge geological basin.
Where to put such a tower was easy enough. There was a site on Cleveland Street that was right next to the Museum Telephone Exchange. Hooking up the tower to the exchange was simply a matter of routing the microwave links straight down into the exchange's switching equipment room. This was not only as direct as possible, it also reduced signal degradation to a minimum.
After the predictable objections that the new tower would ruin the skyline, a team of architects and engineers led by Eric Bedford, chief architect of the Ministry of Public Buildings and Works, along with senior architect G.R. Yeats, set about designing the new building.
In 1961, ground was broken for the project, and the innovations needed to build it became evident immediately. The surveyors learned that there was a thick, sturdy layer of chalk that would be perfect for the foundations. Less perfect was the fact that the chalk sat under 170 ft (52 m) of London blue clay – far too deep to anchor traditional pilings.
Instead, engineers designed a brilliant workaround: they decided to let the tower "float" in the clay. They dug down 24 ft (7.3 m) and poured a concrete raft 89 ft (27 m) on each side and 3 ft (1 m) thick, reinforced with six layers of pre-stressed, post-tensioned steel cables. On top of this sat a reinforced concrete pyramid with a flat top, which distributed the massive load of the 13,000 tonnes of concrete and steel. The structure had to be put together using an entirely new type of tower crane.
The purpose of all this wasn't just to keep the tower from sinking into the ooze; it was to keep the building as absolutely stable as possible. Normally, skyscrapers sway a surprising amount, but the new tower had to be able to transmit and receive precisely aimed microwave beams. That meant it could only shift about 10 inches (25 cm) even in winds reaching 82 knots (95 mph, 153 km/h).
Along with the heavy-duty foundation raft, the tower consisted of a hollow concrete core running the complete height of the building, tapering from 35 ft (11 m) at the base to 22 ft (7 m) at the top, with a corresponding thinning of the core walls. This brought the center of gravity downward, negating the effects of wind shear, as did the round cross-section of the tower.
If you look at the tower from street level, you'll see a wide concrete walkway at 80 ft (24 m) above the ground. It may look like some ghastly Brutalist architectural abomination, but that Bridge Deck connects to the structural steel framework of the adjacent four-story Museum Telephone Exchange, acting as a massive brace to keep the tower structure as still as possible.
On October 8, 1965, the building, now officially named the Post Office Tower, was opened by Prime Minister Harold Wilson. Along with the telecommunications gear, the Tower's 37 floors included an observation deck, a cocktail lounge, and a revolving restaurant. Oddly enough, though floors 1 through 16 looked like offices behind the cladding of specially tinted glass framed in stainless steel, they were actually crammed with electronics and electromechanical relays. As a result, the lower sections of the Tower were almost empty, populated by only a handful of technicians who often had little to do except monitor the telecommunications gear.
Not that the Tower was a tomb – at least, not everywhere. The observation deck attracted 50,000 visitors a week, who could be seen queuing for hours to get in. Meanwhile, the exclusive Top of the Tower restaurant boasted a revolving floor that turned once every 22 minutes, and a special lift to move food from the ground-floor kitchen to the dining room. Diners included the Beatles, Mick Jagger, Muhammad Ali, Lynn Redgrave, and countless other celebrities, politicians, and nobility.
But for all this fame, the Post Office Tower was officially a state secret. This is because it didn't just act as a civilian communications center; it was also the nexus for "Backbone," the classified microwave relay network designed by the GPO and the Ministry of Defence. This system used a series of microwave relay nodes established outside of major towns to act as a vital communications link between strategic government sites, military installations, and regional seats of government in the event of a nuclear strike on Britain.
The locations of these nodes and the frequencies they operated on were, understandably, a closely guarded secret. It also meant that, absurdly, the Tower became the worst-kept secret in history.
This absurdity reached its height in 1978, when journalists and whistleblowers collecting information on state communication networks were put on trial for violating the Official Secrets Act. The judge strictly insisted that the Tower only be referred to in court as "Location 23."
It was no wonder that in 1993, following the end of the Cold War, Member of Parliament Kate Hoey used her parliamentary privilege to finally shatter the illusion. She confirmed the existence of the Tower on the floor of the House of Commons and stated its actual address – 60 Cleveland Street – for the public record. After that, its existence was officially declassified to the surprise of absolutely no one.
That said, I sincerely hope that the Special Branch won't be knocking on my door after this is published to ask a few awkward questions.
Despite its remarkable history, time and events eventually caught up with the Tower. In 1971, a bomb went off in one of the lavatories on the 31st floor. Two entirely different terrorist organizations claimed responsibility, though the actual culprit was never found. The damage was not extensive, but it resulted in the permanent closure of the public observation and dining areas. The restaurant only briefly reopened for private functions until 1980, when the lease ran out.
The year 1980 also saw the Tower relegated to second place in the London skyline when the taller NatWest Tower was completed. In 1984, the GPO's telecommunications division was privatized as British Telecom, and the landmark was renamed the Telecom Tower, and eventually, the BT Tower.
Beyond politics and security, it was technology that finally did for the Tower. The perfection of satellite and fiber-optic communications made microwaves obsolete, while the rapid development of modern computers turned the gubbins on the lower floors into museum pieces. Eventually, even the iconic microwave antennas became safety hazards and were removed.
Today, the Tower stands as a monument to the "White Heat of Technology" phase of British history. Current plans aim to have it converted into a luxury hotel by MCR Hotels by 2033. That might be trickier than it sounds given that the Tower is a Grade II listed building, which means the new owners are severely restricted in terms of what structural renovations they can make. It also doesn't help that the Tower was never designed for large-scale human habitation, presenting serious challenges for modern architects. As a result, the hotel rooms themselves will likely be confined to the podium and the Howland building at the base, rather than inside the column of the Tower itself, unless an extraordinary amount of money is sunk into the project.
When opening the building in 1965, Harold Wilson remarked: "Big Ben represents the fussy grandeur of the Gothic revival that epitomized the Victorian imperial affluence, built on the foundation of the first Industrial Revolution. The Post Office Tower, lean, practical, and futuristic, symbolizes the technical and architectural skill of this new age."
A new age, indeed, but nothing in this world ages faster than the future.
