More investors are becoming wary of a bubble as hyperpriced homes listing for nine figures are just adding up.

According to Christie's International Real Estate, a record number of 27 homes priced at more than $100 million are on the market, up from 19 last year and 12 in 2014. Brokers say that number rises to 40 or 50 homes when 'whisper listings' - homes offered privately - are included

The last time a pileup like this occurred, writes Robert Frank for the New York Times, was in 2007 and 2008 before the crash. In 2008, four homes were listed at $100 million and none of them sold at that price. A mansion in England listed at $138 million sold for just $50 million.

Of course, anyone can slap a $100 million price tag on a home to get attention. Yet actual sales of nine-figure homes are rare, even in good times. Between 2011 and 2016, only 15 homes in the world have sold for $100 million or more, according to Christies, and five of those were in 2014.

Of the 10 most expensive listings in the world, seven are in the United States and four of these are in Los Angeles.

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