The neighbors, Brian May wrote on his blog, had declared war on his family. Beyond the thick blossoms of the cherry tree in his yard towered the corkscrew of a piling rig, pounding into the London clay every morning. May—best known as the lead guitarist from Queen—had had enough. “Kensington used to be a nice quiet decent place to live. Now it is becoming a Hellhole,” he wrote. Not for the first time, he was experiencing the construction of a neighbor’s basement. Time to take action, he concluded.

And his West London borough did. The next year, 2015, Kensington and Chelsea became the first government in the United Kingdom to issue blanket restrictions on the construction of basements. Westminster did the same. In 2016, Islington recommended basements not go deeper than a story. This year, Hammersmith and Fulham enacted a similar policy. The era of the megabasement, a decade during which the West London elite dug like prairie dogs into the sand, gravel, and clay of the Thames Valley, may be coming to an end.

London’s recent transformation into a storehouse for global wealth is usually represented by the Shard, Renzo Piano’s tapering, 95-story skyscraper completed in 2012. But in terms of sheer square footage, more action has been happening below ground, where the city’s historic preservation laws have driven wealthy homeowners looking to carve out extra space. In 2001, according to the Guardian, the borough of Kensington and Chelsea received just a few dozen basement applications; in 2013 it received 450.

The tabloids call them “iceberg homes,” with the finished product invisible save for a couple glowing skylights in the backyard.

In a slice of seven West London boroughs (out of the 32 total that make up London), researchers from Newcastle University found that 4,650 basements had been permitted in the decade ending in 2017. Stacked, they’d reach 50 Shards down into the earth. Most of them go just one story down and lie within the footprint of the house, though even those standard basements in total contain 638 gyms, 278 screening rooms, 52 staff rooms, and seven swimming pools, according to the Newcastle team’s review of planning documents. But it is the almost 900 megabasements that have really rattled teacups in West London’s golden post codes. Swimming pools, gyms, staff rooms, of course, but also car elevators, art galleries, and gun ranges. In many cases, retrieving the excavators doesn’t pencil out for the builders, and so like Mike Mulligan’s steam shovel, hundreds have wound up embedded in the underground landscape they created, a hidden constellation of buried machines beneath the city.

“What we’re doing is like designing and manufacturing a brand-new luxury motorcar, one that’s never been designed or manufactured before, and building it in an old factory,” explains Alan Waxman, the founder of the design studio Landmass London and self-anointed “basement king.” One of Waxman’s projects, a mews house in Belgravia he designed with Nicole Kidman in mind (she didn’t end up moving in), features a 34-foot waterfall tumbling from the second floor down into the basement. The tabloids call them “iceberg homes,” with the finished product invisible save for a couple glowing skylights in the back yard.

Emphasis on finished. In the interim, their construction is an enormous nuisance in neighborhoods that have otherwise been set in amber for generations—especially on blocks where overlapping projects have produced years upon years of construction. That makes good fodder for the London papers, since the frustrated neighbors are often just as rich (and more famous) than the parvenus digging next door. This spring, for example, Led Zeppelin guitarist Jimmy Page appeared at a Kensington planning meeting to plead with the local government to prohibit his next-door neighbor Robbie Williams from building a basement swimming pool. A 2015 BBC special, Millionaire Basement Wars, featured a family whose house was flooded after a dumpster full of excavated dirt took on rainwater, crashed through the street, and smashed a water main. The contractor building the basement had closed down.

A familiar neighborly opposition to hubbub is at play here, a “not under my backyard” sentiment. But the situation is also reminiscent of what has happened in cities like New York and Vancouver, where sky-high housing prices have often been blamed on foreign buyers. As a prominent example, the owner of London’s largest private home—Witanhurst, which counts nearly half of its 90,000 square feet below ground—is the Russian billionaire Andrey Guryev. “There’s a kind of polite racism going on here,” observed Roger Burrows, the professor of cities at Newcastle University who undertook the study of West London basements. “The people who have been complaining the most are what we call the merely wealthy—it’s a battle between the haves and the have-yachts. There’s an awful lot of snobbery, because these things are very vulgar.”

As any counterculture can attest, it’s harder to legislate vulgarity when it happens underground. In an effort to put an end to the construction boom, London boroughs have cited other annoyances, such as traffic, noise, drainage, and structural damage. Some basement builders see it as a ploy: “All of the matters used as reasons for the restrictions were in my view unfounded—a pretense in order to restrict construction activity,” argues Simon Haslam, a managing director at the contractor Basement Force.

One such concern, for example, is that concentrated excavations—like on little Tregunter Road, a 1,000-foot stub where 21 basements have been built in the past decade—might disrupt the way that groundwater moves through the soil, causing rainfall flooding on the surface. This is a frequent complaint that accompanies megabasement applications, such as Damien Hirst’s vision for an 82-foot pool, yoga room, and art gallery beneath his Regent’s Park mansion. Engineers, naturally, dispute that they can’t keep any displaced groundwater in check with pipes and pumps. Basements are big, but London is bigger. “It’s like putting a sugar cube in a bath. The volume of your box is of no consequence,” Haslam says.

A few thousand boxes? That’s less clear. William King, a hydrogeologist at the engineering consultancy WSP, gave a presentation on the issue in April. “How many basements can you fit in without it being overloaded?” he asked. “As an industry, we are looking at that.”

Similarly, while one megabasement doesn’t much to displace the aquifer or change the subterranean structure of a metropolitan area that already has thousands of sewers, water mains, and train tunnels, there are concerns about how isolated “honeycombs” of basements might change the city. Engineers at Transport for London, which runs the Tube, have been investigating a potential link between basement excavations and increased noise and vibrations from passing trains.

One sure effect of the crackdown? Homeowners who built megabasements now have their hands on a rare asset. “It’s really not possible to do that nowadays,” Timothy Comyn, a planning barrister who argued to the councils on behalf of basement builders, observes. “Those houses will be extremely valuable.”