San Francisco's Millennium Tower is sinking and cracks have formed in the building's windows, substantiating data that strongly suggests the city's downtown is slowly sinking. And the Millennium Tower isn't alone -- over the last century, more than three trillion pounds of development and subsequent groundwater loss has caused the Bay Area to sink by three inches.

Why This Matters

Leaning buildings don't just look ominous -- they spell danger. Sea levels rising and ground levels sinking will make the potential for flooding much greater.

US Geological Survey research geophysicist Tom Parsons told NBC: "If you have all of this going on when you're right near the waterline, in some cases in San Francisco, then you have to worry about big storms that sea level comes up and inundation more frequently."

It's not just San Francisco -- cities from Jakarta to Manhattan have been sinking as well.

Why Is the Millennium Tower Tilting?

Since its opening in 2009, the Millennium Tower has sunk approximately 17 inches, and tilted 22 inches to the northwest, according to the Business Insider.

Millennium Tower is only the third-heaviest building in San Francisco at 686 million pounds, but the only one leaning significantly. Therefore sinking is caused by more factors than weight alone. Another significant role is played by groundwater loss from nearby construction and concrete foundations that prevents the tower from reaching bedrock.

But when it comes to sinking buildings, there is still much to learn. Harry Poulos, an internationally recognized expert on tall building foundations, told NBC "I've been doing foundation design for, I guess, nearly 30 years now, and it's not something that we've actually ever even thought about. Not on this sort of scale that you're talking about."