At 58 storeys and 645 feet high (197 metres), the luxury Millennium Tower is the tallest concrete structure in San Francisco. It's also one of the most unstable.

On Saturday, an apartment owner detected a large fissure in his window on the high-rise's 36th floor, according to the San Francisco Chronicle. Earlier that day, residents reported hearing a series of creaking noises, followed by a loud pop.

As of Tuesday, the building's management company had 72 hours to report on the issue. Though officials blocked off part of the footpath, a spokesperson for the Department of Building Inspection said there was no safety risk for pedestrians.

It's concerning news for inhabitants of 301 Mission Street, who have already had to contend with the fact that their building is sinking. In 2016, an independent consultant found the tower had sunk 16 inches (40 centimetres) and tilted 2 inches (5 centimetres) to the northwest since its completion in 2008.

By 2018, it had sunk an additional inch and tilted another 12 inches (30 centimetres). The builders originally anticipated that the structure would sink only 4 to 6 inches (10 to 15 centimetres) over the course of its lifetime.

This rapid shift has led to speculation that the building's facade is separating from its interior, making it vulnerable to an earthquake or fire.

While the Department of Building Inspection has since determined that the tower is safe to live in, residents are concerned about their investments.

When the building first opened, its units sold for anywhere from US$1.6 million to more than US$10 million. Since then, around 100 condo owners have each seen a US$320,000 drop on average in their apartment value.

The high-rise has also endured a string of lawsuits, including one spearheaded by the homeowners' association.

Developers have gone back and forth over who's to blame.

Millennium Partners, the real estate company behind the tower, cites the 2010 construction of the recently-completed Salesforce Transit Center as the reason for the sinking.

The developers contend that construction workers pumped too much water out of the ground while the transit center was being built, causing the sand to compress and the tower to settle.

But the Transbay Joint Powers Authority, which oversaw the center's development, argue that the sinking started before they broke ground.

Earlier this year, engineers proposed a solution to the tower's structural instability: a project with a price tag up to US$500 million that involves anchoring the building to bedrock, which is more compact and tends to shake less than sand.

Until then, residents may continue to witness unsettling damages, such as cracks in the basement or malfunctioning elevators, as the tower continues to shift.

This article was originally published by Business Insider.

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