One multi-million-dollar London townhouse wasn’t enough, so they bought the multi-million-dollar London townhouse next door to knock them together.

Still, that wasn’t enough.

So they called in contractors to dig out a huge basement to extend the living space of the fine Chelsea Georgian homes below ground—a trend that is rampant in London’s richest neighborhoods where the wealthy compete to build the most lavish subterranean gyms, pools, home theaters, and even showrooms for their collections of classic cars.

On Monday night, one such extravagant basement dream turned into a nightmare: a $13-million sinkhole in one of London’s most expensive and desirable residential streets. The two townhouses that were being subjected to extensive basement works collapsed into rubble.

Knocking the two houses together ticks another box of the deeply annoying millionaire property trends in London and New York.

For ordinary Londoners, many of whom persistently complain that the bone-shaking excavations involved in creating luxurious mega-basements ruin their lives (and the structural integrity of their own homes) the sense of justice will be poetic indeed.

Any gloating by residents of the street itself, Durham Place in Kensington, however, is likely to be tempered by the fact that some 40 people living mainly in apartments adjoining the fallen building have now been evacuated from their homes as a result of the collapse.

The London Evening Standard reports that, according to planning documents submitted to Kensington and Chelsea council, some of the properties in the street have approval for a gym, media room, and study to be created in their basements.

Neighbors reported hearing a huge thud “as loud as thunder” in the middle of the night as the mid-terraced houses, worth about £10 million ($13 million), came tumbling down.

Emergency services and fire crews worked through the night to secure the buildings.

The Fire Brigade’s Jason Jones said, “There was a total collapse of the building from the roof to ground level. Firefighters worked to make the scene safe and our drone team carried out a search.

“A police search dog also carried out an external search of the building, and at this stage, there are no reports of any injuries. Nobody is thought to have been inside the building at the time of the collapse.”

While the trend for super-basements has become a well-known peril of contemporary London life in particular, plutocratic New Yorkers have also gotten in on the act: a $100-million brownstone on West 69th Street between Central Park West and Columbus Avenue was leveled and is being rebuilt with a basement 38 feet deep to provide for an underground theater and recording studio.